Allele-Specific H3K79 Di- versus Trimethylation Distinguishes Opposite Parental Alleles at Imprinted Regions

Imprinted gene expression corresponds to parental allele-specific DNA CpG methylation and chromatin composition. Histone tail covalent modifications have been extensively studied, but it is not known whether modifications in the histone globular domains can also discriminate between the parental alleles. Using multiplex chromatin immunoprecipitation-single nucleotide primer extension (ChIP-SNuPE) assays, we measured the allele-specific enrichment of H3K79 methylation and H4K91 acetylation along the H19/Igf2 imprinted domain. Whereas H3K79me1, H3K79me2, and H4K91ac displayed a paternal-specific enrichment at the paternally expressed Igf2 locus, H3K79me3 was paternally biased at the maternally expressed H19 locus, including the paternally methylated imprinting control region (ICR). We found that these allele-specific differences depended on CTCF binding in the maternal ICR allele. We analyzed an additional 11 differentially methylated regions (DMRs) and found that, in general, H3K79me3 was associated with the CpG-methylated alleles, whereas H3K79me1, H3K79me2, and H4K91ac enrichment was specific to the unmethylated alleles. Our data suggest that allele-specific differences in the globular histone domains may constitute a layer of the “histone code” at imprinted genes.Imprinted genes are defined by the characteristic monoallelic silencing of either the paternally or maternally inherited allele. Most imprinted genes exist in imprinted gene clusters (10), and these clusters are usually associated with one or more differentially methylated regions (DMRs) (27, 65). DNA methylation at DMRs is essential for the allele-specific expression of most imprinted genes (31). Maternal or paternal allele-specific DNA methylation of a subset of DMRs (germ line DMRs) is gamete specific (27, 39). These maternal or paternal methylation differences are established during oogenesis or spermatogenesis, respectively, by the de novo DNA methyltransferases Dnmt3a and Dnmt3b together with Dnmt3L (5, 26, 48). The gamete-specific methylation differences set the stage for the parental allele-specific action of germ line DMRs, some of which have been shown to control the monoallelic expression of the associated genes in the respective domains (11, 34, 36, 53, 66, 71-73, 77). These DMRs are called imprinting control regions (ICRs).Two recurring themes have been reported for ICR action. ICRs can function as DNA methylation-regulated promoters of a noncoding RNA or as methylation-regulated insulators. Recent evidence suggests that both of these mechanisms involve chromatin organization by either the noncoding RNA (45, 50) or the CTCF insulator protein (17, 32) along the respective imprinted domains. The CTCF insulator binds in the unmethylated maternal allele of the H19/Igf2 ICR and blocks the access of the Igf2 promoters to the shared downstream enhancers. CTCF cannot bind in the methylated paternal ICR allele; hence, here the Igf2 promoters have access to the enhancers (4, 18, 24, 25, 62). When CTCF binding is abolished in the ICR of the maternal allele, Igf2 expression becomes biallelic, and H19 expression is missing from both alleles (17, 52, 58, 63). Importantly, CTCF is the single major organizer of the allele-specific chromatin along the H19/Igf2 imprinted domain (17). Significantly, CTCF recruits, at a distance, Polycomb-mediated H3K27me3 repressive marks at the Igf2 promoter and at the Igf2 DMRs (17, 32).A role for chromatin composition is suggested in the parental allele-specific expression of imprinted genes. Repressive histone tail covalent modifications, such as H3K9me2 H3K9me3, H4K20me3, H3K27me3, and the symmetrically methylated H4R3me2 marks, are generally associated with the methylated DMR alleles, while activating histone tail covalent modifications, such as acetylated histone tails and also H3K4me2 and H3K4me3, are characteristic of the unmethylated alleles (7-9, 12-15, 17, 21, 33, 35, 43, 44, 51, 55, 56, 67, 69, 74, 75). Importantly, the maintenance of imprinted gene expression depends on the allele-specific chromatin differences. ICR-dependent H3K9me2 and H3K27me3 enrichment in the paternal allele (67) is required for paternal repression of a set of imprinted genes along the Kcnq1 imprinted domain in the placenta (30). Imprinted Cdkn1c and Cd81 expression depends on H3K27 methyltransferase Ezh2 activity in the extraembryonic ectoderm (64). Similarly, H3K9 methyltransferase Ehmt2 is required for parental allele-specific expression of a number of imprinted genes, including Osbpl5, Cd81, Ascl2, Tfpi2, and Slc22a3 in the placenta (44, 45, 70).There is increasing evidence that covalent modifications, not only in the histone tails but also in the histone globular domains, carry essential information for development and gene regulation. The H3K79 methyltransferase gene is essential for development in Drosophila (60) and in mice (22). H3K79 methylation is required for telomeric heterochromatin silencing in Drosophila (60), Saccharomyces cerevisiae (47, 68), and mice (22). The H4K91 residue regulates nucleosome assembly (76). Whereas mutations at single acetylation sites in the histone tails have only minor consequences, mutation of the H4K91 site in the histone H4 globular domain causes severe defects in silent chromatin formation and DNA repair in yeast (37, 42, 76).Contrary to the abundant information that exists regarding the allele-specific chromatin composition at DMRs of imprinted genes, no information is available about the parental allele-specific marking in the histone globular domains at the DMRs. We hypothesized that chromatin marks in the globular domains of histones also distinguish the parental alleles of germ line DMRs. In order to demonstrate this, we measured the allele-specific enrichment of H3K79me1, H3K79me2, H3K79me3, and H4K91ac at 11 mouse DMRs using quantitative multiplex chromatin immunoprecipitation-single nucleotide primer extension (ChIP-SNuPE) assays. In general, H3K79me3 was associated with the methylated allele at most DMRs, whereas the unmethylated allele showed enrichment for H3K79me1, H3K79me2, and H4K91ac. These results are consistent with the possibility that allele-specific differences in the globular domains of histones contribute to the “histone code” at DMRs.     

The Imprinted Phlda2 Gene Regulates Extraembryonic Energy Stores

An important difference between placental mammals and marsupials is the maturity of the fetus at birth. Placental mammals achieved this maturity by developing a complex and invasive placenta to support and prolong internal development. The exact genomic modifications that facilitated the evolution of this complex structure are unknown, but the emergence of genomic imprinting within mammalian lineages suggests a role for gene dosage. Here we show that a maximally altered placental structure is achieved by a single extra dose of the imprinted Phlda2 gene characterized by a dramatically reduced junctional zone and a decrease in stored glycogen. In addition, glycogen cells do not migrate into the maternal decidua in a timely fashion, but instead, Tpbpa-positive cells progressively mislocalize into the labyrinth. These defects are linked to a progressive restriction of embryonic growth from embryonic day 16.5. This work has identified a critical role for the imprinted Phlda2 gene in regulating glycogen storage in the eutherian placenta and implies that imprinting has provided a mechanism to boost nutrient supply to the fetus late in gestation, when the fetus is placing the highest demands on maternal resources, to enhance growth.Distinct to mammals, embryonic growth is dependent on the ability of the mother to support in utero growth. The choriovitelline placenta initially provides access to maternal nutrients, and, as the demands of fetal growth increase, monotremes and marsupials remain dependent on the yolk sac placenta but eutherian mammals switch to an elaborate chorioallantoic placenta (22, 43). Very few genes are expressed uniquely in the placenta. The majority have arisen from existing genes by means of placenta-specific promoters, from the duplication of large gene families, or through the adoption of functions associated with endogenous retroviruses and retroelements (42). A surprising number of imprinted gene knockout models exhibit placental defects (19), suggesting gene dosage as another mechanism important in the evolution of the fetoplacental unit. Approximately 0.3% of autosomal genes are imprinted in eutherian mammals, while a subset of these genes are imprinted in marsupials with no evidence of imprinting in other vertebrates (1, 31, 32, 37, 39, 51, 54, 56, 58). Thus, the emergence of genomic imprinting coincides with the appearance of extraembryonic support, and, as the demands for this support have increased, the number of imprinted genes co-opted by the imprinting mechanism has increased (30), also suggesting the involvement of these unique genes in placental development.The mouse placenta is organized into the histologically distinct labyrinth zone, junctional zone, giant cell layer, and maternal decidua (9-11, 27, 45, 49). The giant cells are thought to modify the maternal uterine vasculature, promoting maternal blood flow toward the implantation site, while in the labyrinth zone exchange takes place between the maternal and fetal circulation. The junctional zone, also known as the spongiotrophoblast layer, provides a source of pregnancy-related hormones (9, 35), but, although this layer is absolutely required for embryonic survival (25, 26), its function is less well understood. It is composed of two major cell types, spongiotrophoblast and glycogen cells, which both express trophoblast-specific protein alpha (Tpbpa), with the glycogen cells additionally accumulating glycogen within their cytoplasm from embryonic day 12.5 (E12.5) (5, 9). An unusual feature of glycogen cells is their migration into the maternal decidua late in gestation, where they may function to provide a rapidly mobilizable energy source during late pregnancy and parturition. Despite the amazing variety in the forms and types of eutherian placenta, easily detectable stores of glycogen are a common feature (8).Imprinted genes located at mouse distal chromosome 7 play an important role in regulating embryonic and placental growth (16, 38). With regard to imprinting, this chromosomal region can be separated mechanistically into two distinct domains (7). Each domain contains one key gene that directly modulates embryonic growth. The IC1 domain contains the gene for the potent embryonic growth factor insulin-like growth factor 2 (Igf2) (13, 14). Global loss of expression of Igf2 directly limits embryonic growth, while Igf2 deficiency localized to the placenta indirectly restricts embryonic growth (12). The predicted consequence of imprinting Igf2 (reduced dosage) would be to limit embryonic growth. Cyclin-dependent kinase inhibitor 1C (Cdkn1c) is the major regulator of embryonic growth within the adjacent IC2 domain (2). In contrast to Igf2, imprinting of Cdkn1c would be predicted to enhance embryonic growth. Pleckstrin homology-like domain family A member 2 (Phlda2) and achaete-scute complex homolog 2 (Ascl2) also map to the IC2 region (24, 41) but primarily play a role in extraembryonic development. Ascl2 deficiency results in embryonic lethality at midgestation due to placental failure, but tetraploid rescue experiments exclude a direct role in regulating embryonic growth or adult development (25, 26, 53). Phlda2 is also predominantly expressed in the placenta from the maternal allele being expressed in syncytiotrophoblast layers II and III of the labyrinth (15, 21, 41). Phlda2 deficiency results in placentomegaly with a specific increase in the area of the junctional zone but with no overt consequence for embryonic growth or adult development (20).A mouse model of loss of imprinting of the IC2 domain, in which several imprinted genes are overexpressed, shows placental stunting (17) and a reduction of the junctional zone (46). We previously showed, indirectly, that Phlda2 rescues the volume of the junctional zone by normalizing Phlda2 expression in these Kvdmr1^+/− mice. We also showed that excess dosage of the region spanning Phlda2 and a second imprinted gene, the solute carrier family 22 member 18 gene (Slc22a18), restricts placental growth and noted a subtle and late embryonic growth restriction phenotype (46). In our transgenic model the two imprinted genes were overexpressed at high levels from three copies of a bacterial artificial chromosome (BAC), suggesting misregulated expression. Given the importance of the junctional zone in embryonic viability and the potential role of PHLDA2 in human intrauterine growth restriction (IUGR) (36), we sought to perform a more detailed characterization of the consequence of excess expression of Phlda2 and Slc22a18 in three independent transgenic lines with increasing doses of the transgene and in two genetic backgrounds. Using a single-copy transgene, we asked whether normalizing Phlda2 expression rescued the identified phenotypes, which included a unique mislocalization defect. Finally, we characterized embryonic growth from E13.5 to E18.5 in two independent lines. We identify critical roles for Phlda2 in regulating glycogen storage and in coordinating the location of spongiotrophoblast and glycogen cells late in gestation.     

A New Borrelia Species Defined by Multilocus Sequence Analysis of Housekeeping Genes

Analysis of Lyme borreliosis (LB) spirochetes, using a novel multilocus sequence analysis scheme, revealed that OspA serotype 4 strains (a rodent-associated ecotype) of Borrelia garinii were sufficiently genetically distinct from bird-associated B. garinii strains to deserve species status. We suggest that OspA serotype 4 strains be raised to species status and named Borrelia bavariensis sp. nov. The rooted phylogenetic trees provide novel insights into the evolutionary history of LB spirochetes.Multilocus sequence typing (MLST) and multilocus sequence analysis (MLSA) have been shown to be powerful and pragmatic molecular methods for typing large numbers of microbial strains for population genetics studies, delineation of species, and assignment of strains to defined bacterial species (4, 13, 27, 40, 44). To date, MLST/MLSA schemes have been applied only to a few vector-borne microbial populations (1, 6, 30, 37, 40, 41, 47).Lyme borreliosis (LB) spirochetes comprise a diverse group of zoonotic bacteria which are transmitted among vertebrate hosts by ixodid (hard) ticks. The most common agents of human LB are Borrelia burgdorferi (sensu stricto), Borrelia afzelii, Borrelia garinii, Borrelia lusitaniae, and Borrelia spielmanii (7, 8, 12, 35). To date, 15 species have been named within the group of LB spirochetes (6, 31, 32, 37, 38, 41). While several of these LB species have been delineated using whole DNA-DNA hybridization (3, 20, 33), most ecological or epidemiological studies have been using single loci (5, 9-11, 29, 34, 36, 38, 42, 51, 53). Although some of these loci have been convenient for species assignment of strains or to address particular epidemiological questions, they may be unsuitable to resolve evolutionary relationships among LB species, because it is not possible to define any outgroup. For example, both the 5S-23S intergenic spacer (5S-23S IGS) and the gene encoding the outer surface protein A (ospA) are present only in LB spirochete genomes (36, 43). The advantage of using appropriate housekeeping genes of LB group spirochetes is that phylogenetic trees can be rooted with sequences of relapsing fever spirochetes. This renders the data amenable to detailed evolutionary studies of LB spirochetes.LB group spirochetes differ remarkably in their patterns and levels of host association, which are likely to affect their population structures (22, 24, 46, 48). Of the three main Eurasian Borrelia species, B. afzelii is adapted to rodents, whereas B. valaisiana and most strains of B. garinii are maintained by birds (12, 15, 16, 23, 26, 45). However, B. garinii OspA serotype 4 strains in Europe have been shown to be transmitted by rodents (17, 18) and, therefore, constitute a distinct ecotype within B. garinii. These strains have also been associated with high pathogenicity in humans, and their finer-scale geographical distribution seems highly focal (10, 34, 52, 53).In this study, we analyzed the intra- and interspecific phylogenetic relationships of B. burgdorferi, B. afzelii, B. garinii, B. valaisiana, B. lusitaniae, B. bissettii, and B. spielmanii by means of a novel MLSA scheme based on chromosomal housekeeping genes (30, 48).     

Cultivation of Fastidious Bacteria by Viability Staining and Micromanipulation in a Soil Substrate Membrane System

Soil substrate membrane systems allow for microcultivation of fastidious soil bacteria as mixed microbial communities. We isolated established microcolonies from these membranes by using fluorescence viability staining and micromanipulation. This approach facilitated the recovery of diverse, novel isolates, including the recalcitrant bacterium Leifsonia xyli, a plant pathogen that has never been isolated outside the host.The majority of bacterial species have never been recovered in the laboratory (1, 14, 19, 24). In the last decade, novel cultivation approaches have successfully been used to recover “unculturables” from a diverse range of divisions (23, 25, 29). Most strategies have targeted marine environments (4, 23, 25, 32), but soil offers the potential for the investigation of vast numbers of undescribed species (20, 29). Rapid advances have been made toward culturing soil bacteria by reformulating and diluting traditional media, extending incubation times, and using alternative gelling agents (8, 21, 29).The soil substrate membrane system (SSMS) is a diffusion chamber approach that uses extracts from the soil of interest as the growth substrate, thereby mimicking the environment under investigation (12). The SSMS enriches for slow-growing oligophiles, a proportion of which are subsequently capable of growing on complex media (23, 25, 27, 30, 32). However, the SSMS results in mixed microbial communities, with the consequent difficulty in isolation of individual microcolonies for further characterization (10).Micromanipulation has been widely used for the isolation of specific cell morphotypes for downstream applications in molecular diagnostics or proteomics (5, 15). This simple technology offers the opportunity to select established microcolonies of a specific morphotype from the SSMS when combined with fluorescence visualization (3, 11). Here, we have combined the SSMS, fluorescence viability staining, and advanced micromanipulation for targeted isolation of viable, microcolony-forming soil bacteria.     

Alignment of the c-Type Cytochrome OmcS along Pili of Geobacter sulfurreducens

Immunogold localization revealed that OmcS, a cytochrome that is required for Fe(III) oxide reduction by Geobacter sulfurreducens, was localized along the pili. The apparent spacing between OmcS molecules suggests that OmcS facilitates electron transfer from pili to Fe(III) oxides rather than promoting electron conduction along the length of the pili.There are multiple competing/complementary models for extracellular electron transfer in Fe(III)- and electrode-reducing microorganisms (8, 18, 20, 44). Which mechanisms prevail in different microorganisms or environmental conditions may greatly influence which microorganisms compete most successfully in sedimentary environments or on the surfaces of electrodes and can impact practical decisions on the best strategies to promote Fe(III) reduction for bioremediation applications (18, 19) or to enhance the power output of microbial fuel cells (18, 21).The three most commonly considered mechanisms for electron transfer to extracellular electron acceptors are (i) direct contact between redox-active proteins on the outer surfaces of the cells and the electron acceptor, (ii) electron transfer via soluble electron shuttling molecules, and (iii) the conduction of electrons along pili or other filamentous structures. Evidence for the first mechanism includes the necessity for direct cell-Fe(III) oxide contact in Geobacter species (34) and the finding that intensively studied Fe(III)- and electrode-reducing microorganisms, such as Geobacter sulfurreducens and Shewanella oneidensis MR-1, display redox-active proteins on their outer cell surfaces that could have access to extracellular electron acceptors (1, 2, 12, 15, 27, 28, 31-33). Deletion of the genes for these proteins often inhibits Fe(III) reduction (1, 4, 7, 15, 17, 28, 40) and electron transfer to electrodes (5, 7, 11, 33). In some instances, these proteins have been purified and shown to have the capacity to reduce Fe(III) and other potential electron acceptors in vitro (10, 13, 29, 38, 42, 43, 48, 49).Evidence for the second mechanism includes the ability of some microorganisms to reduce Fe(III) that they cannot directly contact, which can be associated with the accumulation of soluble substances that can promote electron shuttling (17, 22, 26, 35, 36, 47). In microbial fuel cell studies, an abundance of planktonic cells and/or the loss of current-producing capacity when the medium is replaced is consistent with the presence of an electron shuttle (3, 14, 26). Furthermore, a soluble electron shuttle is the most likely explanation for the electrochemical signatures of some microorganisms growing on an electrode surface (26, 46).Evidence for the third mechanism is more circumstantial (19). Filaments that have conductive properties have been identified in Shewanella (7) and Geobacter (41) species. To date, conductance has been measured only across the diameter of the filaments, not along the length. The evidence that the conductive filaments were involved in extracellular electron transfer in Shewanella was the finding that deletion of the genes for the c-type cytochromes OmcA and MtrC, which are necessary for extracellular electron transfer, resulted in nonconductive filaments, suggesting that the cytochromes were associated with the filaments (7). However, subsequent studies specifically designed to localize these cytochromes revealed that, although the cytochromes were extracellular, they were attached to the cells or in the exopolymeric matrix and not aligned along the pili (24, 25, 30, 40, 43). Subsequent reviews of electron transfer to Fe(III) in Shewanella oneidensis (44, 45) appear to have dropped the nanowire concept and focused on the first and second mechanisms.Geobacter sulfurreducens has a number of c-type cytochromes (15, 28) and multicopper proteins (12, 27) that have been demonstrated or proposed to be on the outer cell surface and are essential for extracellular electron transfer. Immunolocalization and proteolysis studies demonstrated that the cytochrome OmcB, which is essential for optimal Fe(III) reduction (15) and highly expressed during growth on electrodes (33), is embedded in the outer membrane (39), whereas the multicopper protein OmpB, which is also required for Fe(III) oxide reduction (27), is exposed on the outer cell surface (39).OmcS is one of the most abundant cytochromes that can readily be sheared from the outer surfaces of G. sulfurreducens cells (28). It is essential for the reduction of Fe(III) oxide (28) and for electron transfer to electrodes under some conditions (11). Therefore, the localization of this important protein was further investigated.     

Virus Entry via the Alternative Coreceptors CCR3 and FPRL1 Differs by Human Immunodeficiency Virus Type 1 Subtype

Human immunodeficiency virus type 1 (HIV-1) infects target cells by binding to CD4 and a chemokine receptor, most commonly CCR5. CXCR4 is a frequent alternative coreceptor (CoR) in subtype B and D HIV-1 infection, but the importance of many other alternative CoRs remains elusive. We have analyzed HIV-1 envelope (Env) proteins from 66 individuals infected with the major subtypes of HIV-1 to determine if virus entry into highly permissive NP-2 cell lines expressing most known alternative CoRs differed by HIV-1 subtype. We also performed linear regression analysis to determine if virus entry via the major CoR CCR5 correlated with use of any alternative CoR and if this correlation differed by subtype. Virus pseudotyped with subtype B Env showed robust entry via CCR3 that was highly correlated with CCR5 entry efficiency. By contrast, viruses pseudotyped with subtype A and C Env proteins were able to use the recently described alternative CoR FPRL1 more efficiently than CCR3, and use of FPRL1 was correlated with CCR5 entry. Subtype D Env was unable to use either CCR3 or FPRL1 efficiently, a unique pattern of alternative CoR use. These results suggest that each subtype of circulating HIV-1 may be subject to somewhat different selective pressures for Env-mediated entry into target cells and suggest that CCR3 may be used as a surrogate CoR by subtype B while FPRL1 may be used as a surrogate CoR by subtypes A and C. These data may provide insight into development of resistance to CCR5-targeted entry inhibitors and alternative entry pathways for each HIV-1 subtype.Human immunodeficiency virus type 1 (HIV-1) infects target cells by binding first to CD4 and then to a coreceptor (CoR), of which C-C chemokine receptor 5 (CCR5) is the most common (6, 53). CXCR4 is an additional CoR for up to 50% of subtype B and D HIV-1 isolates at very late stages of disease (4, 7, 28, 35). Many other seven-membrane-spanning G-protein-coupled receptors (GPCRs) have been identified as alternative CoRs when expressed on various target cell lines in vitro, including CCR1 (76, 79), CCR2b (24), CCR3 (3, 5, 17, 32, 60), CCR8 (18, 34, 38), GPR1 (27, 65), GPR15/BOB (22), CXCR5 (39), CXCR6/Bonzo/STRL33/TYMSTR (9, 22, 25, 45, 46), APJ (26), CMKLR1/ChemR23 (49, 62), FPLR1 (67, 68), RDC1 (66), and D6 (55). HIV-2 and simian immunodeficiency virus SIVmac isolates more frequently show expanded use of these alternative CoRs than HIV-1 isolates (12, 30, 51, 74), and evidence that alternative CoRs other than CXCR4 mediate infection of primary target cells by HIV-1 isolates is sparse (18, 30, 53, 81). Genetic deficiency in CCR5 expression is highly protective against HIV-1 transmission (21, 36), establishing CCR5 as the primary CoR. The importance of alternative CoRs other than CXCR4 has remained elusive despite many studies (1, 30, 70, 81). Expansion of CoR use from CCR5 to include CXCR4 is frequently associated with the ability to use additional alternative CoRs for viral entry (8, 16, 20, 63, 79) in most but not all studies (29, 33, 40, 77, 78). This finding suggests that the sequence changes in HIV-1 env required for use of CXCR4 as an additional or alternative CoR (14, 15, 31, 37, 41, 57) are likely to increase the potential to use other alternative CoRs.We have used the highly permissive NP-2/CD4 human glioma cell line developed by Soda et al. (69) to classify virus entry via the alternative CoRs CCR1, CCR3, CCR8, GPR1, CXCR6, APJ, CMKLR1/ChemR23, FPRL1, and CXCR4. Full-length molecular clones of 66 env genes from most prevalent HIV-1 subtypes were used to generate infectious virus pseudotypes expressing a luciferase reporter construct (19, 57). Two types of analysis were performed: the level of virus entry mediated by each alternative CoR and linear regression of entry mediated by CCR5 versus all other alternative CoRs. We thus were able to identify patterns of alternative CoR use that were subtype specific and to determine if use of any alternative CoR was correlated or independent of CCR5-mediated entry. The results obtained have implications for the evolution of env function, and the analyses revealed important differences between subtype B Env function and all other HIV-1 subtypes.     

Abundance of Novel and Diverse tfdA-Like Genes,Encoding Putative Phenoxyalkanoic Acid Herbicide-Degrading Dioxygenases,in Soil

Phenoxyalkanoic acid (PAA) herbicides are widely used in agriculture. Biotic degradation of such herbicides occurs in soils and is initiated by α-ketoglutarate- and Fe²⁺-dependent dioxygenases encoded by tfdA-like genes (i.e., tfdA and tfdAα). Novel primers and quantitative kinetic PCR (qPCR) assays were developed to analyze the diversity and abundance of tfdA-like genes in soil. Five primer sets targeting tfdA-like genes were designed and evaluated. Primer sets 3 to 5 specifically amplified tfdA-like genes from soil, and a total of 437 sequences were retrieved. Coverages of gene libraries were 62 to 100%, up to 122 genotypes were detected, and up to 389 genotypes were predicted to occur in the gene libraries as indicated by the richness estimator Chao1. Phylogenetic analysis of in silico-translated tfdA-like genes indicated that soil tfdA-like genes were related to those of group 2 and 3 Bradyrhizobium spp., Sphingomonas spp., and uncultured soil bacteria. Soil-derived tfdA-like genes were assigned to 11 clusters, 4 of which were composed of novel sequences from this study, indicating that soil harbors novel and diverse tfdA-like genes. Correlation analysis of 16S rRNA and tfdA-like gene similarity indicated that any two bacteria with D > 20% of group 2 tfdA-like gene-derived protein sequences belong to different species. Thus, data indicate that the soil analyzed harbors at least 48 novel bacterial species containing group 2 tfdA-like genes. Novel qPCR assays were established to quantify such new tfdA-like genes. Copy numbers of tfdA-like genes were 1.0 × 10⁶ to 65 × 10⁶ per gram (dry weight) soil in four different soils, indicating that hitherto-unknown, diverse tfdA-like genes are abundant in soils.Phenoxyalkanoic acid (PAA) herbicides such as MCPA (4-chloro-2-methyl-phenoxyacetic acid) and 2,4-D (2,4-dichlorophenoxyacetic acid) are widely used to control broad-leaf weeds in agricultural as well as nonagricultural areas (19, 77). Degradation occurs primarily under oxic conditions in soil, and microorganisms play a key role in the degradation of such herbicides in soil (62, 64). Although relatively rapidly degraded in soil (32, 45), both MCPA and 2,4-D are potential groundwater contaminants (10, 56, 70), accentuating the importance of bacterial PAA herbicide-degrading bacteria in soils (e.g., references 3, 5, 6, 20, 41, 59, and 78).Degradation can occur cometabolically or be associated with energy conservation (15, 54). The first step in the degradation of 2,4-D and MCPA is initiated by the product of cadAB or tfdA-like genes (29, 30, 35, 67), which constitutes an α-ketoglutarate (α-KG)- and Fe²⁺-dependent dioxygenase. TfdA removes the acetate side chain of 2,4-D and MCPA to produce 2,4-dichlorophenol and 4-chloro-2-methylphenol, respectively, and glyoxylate while oxidizing α-ketoglutarate to CO₂ and succinate (16, 17).Organisms capable of PAA herbicide degradation are phylogenetically diverse and belong to the Alpha-, Beta-, and Gammproteobacteria and the Bacteroidetes/Chlorobi group (e.g., references 2, 14, 29-34, 39, 60, 68, and 71). These bacteria harbor tfdA-like genes (i.e., tfdA or tfdAα) and are categorized into three groups on an evolutionary and physiological basis (34). The first group consists of beta- and gammaproteobacteria and can be further divided into three distinct classes based on their tfdA genes (30, 46). Class I tfdA genes are closely related to those of Cupriavidus necator JMP134 (formerly Ralstonia eutropha). Class II tfdA genes consist of those of Burkholderia sp. strain RASC and a few strains that are 76% identical to class I tfdA genes. Class III tfdA genes are 77% identical to class I and 80% identical to class II tfdA genes and linked to MCPA degradation in soil (3). The second group consists of alphaproteobacteria, which are closely related to Bradyrhizobium spp. with tfdAα genes having 60% identity to tfdA of group 1 (18, 29, 34). The third group also harbors the tfdAα genes and consists of Sphingomonas spp. within the alphaproteobacteria (30).Diverse PAA herbicide degraders of all three groups were identified in soil by cultivation-dependent studies (32, 34, 41, 78). Besides CadAB, TfdA and certain TfdAα proteins catalyze the conversion of PAA herbicides (29, 30, 35). All groups of tfdA-like genes are potentially linked to the degradation of PAA herbicides, although alternative primary functions of group 2 and 3 TfdAs have been proposed (30, 35). However, recent cultivation-independent studies focused on 16S rRNA genes or solely on group 1 tfdA sequences in soil (e.g., references 3-5, 13, and 41). Whether group 2 and 3 tfdA-like genes are also quantitatively linked to the degradation of PAA herbicides in soils is unknown. Thus, tools to target a broad range of tfdA-like genes are needed to resolve such an issue. Primers used to assess the diversity of tfdA-like sequences used in previous studies were based on the alignment of approximately 50% or less of available sequences to date (3, 20, 29, 32, 39, 47, 58, 73). Primers specifically targeting all major groups of tfdA-like genes to assess and quantify a broad diversity of potential PAA degraders in soil are unavailable. Thus, the objectives of this study were (i) to develop primers specific for all three groups of tfdA-like genes, (ii) to establish quantitative kinetic PCR (qPCR) assays based on such primers for different soil samples, and (iii) to assess the diversity and abundance of tfdA-like genes in soil.     

Detection,Distribution, and Organohalogen Compound Discovery Implications of the Reduced Flavin Adenine Dinucleotide-Dependent Halogenase Gene in Major Filamentous Actinomycete Taxonomic Groups

Halogenases have been shown to play a significant role in biosynthesis and introducing the bioactivity of many halogenated secondary metabolites. In this study, 54 reduced flavin adenine dinucleotide (FADH₂)-dependent halogenase gene-positive strains were identified after the PCR screening of a large collection of 228 reference strains encompassing all major families and genera of filamentous actinomycetes. The wide distribution of this gene was observed to extend to some rare lineages with higher occurrences and large sequence diversity. Subsequent phylogenetic analyses revealed that strains containing highly homologous halogenases tended to produce halometabolites with similar structures, and halogenase genes are likely to propagate by horizontal gene transfer as well as vertical inheritance within actinomycetes. Higher percentages of halogenase gene-positive strains than those of halogenase gene-negative ones contained polyketide synthase genes and/or nonribosomal peptide synthetase genes or displayed antimicrobial activities in the tests applied, indicating their genetic and physiological potentials for producing secondary metabolites. The robustness of this halogenase gene screening strategy for the discovery of particular biosynthetic gene clusters in rare actinomycetes besides streptomycetes was further supported by genome-walking analysis. The described distribution and phylogenetic implications of the FADH₂-dependent halogenase gene present a guide for strain selection in the search for novel organohalogen compounds from actinomycetes.It is well known that actinomycetes, notably filamentous actinomycetes, have a remarkable capacity to produce bioactive molecules for drug development (4, 6). However, novel technologies are demanded for the discovery of new bioactive secondary metabolites from these microbes to meet the urgent medical need for drug candidates (5, 9, 31).Genome mining recently has been used to search for new drug leads (7, 20, 42, 51). Based on the hypothesis that secondary metabolites with similar structures are biosynthesized by gene clusters that harbor certain homologous genes, such homologous genes could serve as suitable markers for distinct natural-product gene clusters (26, 51). A wide range of structurally diverse bioactive compounds are synthesized by polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) systems in actinomycetes, therefore much attention has been given to revealing a previously unrecognized biosynthetic potential of actinomycetes through the genome mining of these genes (2, 3, 22). However, the broad distribution of PKS and NRPS genes and their high numbers even in a single actinomycete complicate their use (2, 3). To rationally exploit the genetic potential of actinomycetes, more and more special genes, such as tailoring enzyme genes, are being utilized for this sequence-guided genetic screening strategy (20, 38).Tailoring enzymes, which are responsible for the introduction and generation of diversity and bioactivity in several structural classes during or after NRPS, PKS, or NRPS/PKS assembly lines, usually include acyltransferases, aminotransferases, cyclases, glycosyltransferases, halogenases, ketoreductases, methyltransferases, and oxygenases (36, 45). Halogenation, an important feature for the bioactivity of a large number of distinct natural products (16, 18, 30), frequently is introduced by one type of halogenase, called reduced flavin adenine dinucleotide (FADH₂)-dependent (or flavin-dependent) halogenase (10, 12, 35). More than 4,000 halometabolites have been discovered (15), including commercially important antibiotics such as chloramphenicol, vancomycin, and teicoplanin (43).Previous investigations of FADH₂-dependent halogenase genes were focused largely on related gene clusters in the genera Amycolatopsis (33, 44, 53) and Streptomyces (8, 10, 21, 27, 32, 34, 47-49) and also on those in the genera Actinoplanes (25), Actinosynnema (50), Micromonospora (1), and Nonomuraea (39); however, none of these studies has led to the rest of the major families and genera of actinomycetes. In addition, there is evidence that FADH₂-dependent halogenase genes of streptomycetes usually exist in halometabolite biosynthetic gene clusters (20), but we lack knowledge of such genes and clusters in other actinomycetes.In the present study, we show that the distribution of the FADH₂-dependent halogenase gene in filamentous actinomycetes does indeed correlate with the potential for halometabolite production based on other genetic or physiological factors. We also showed that genome walking near the halogenase gene locus could be employed to identify closely linked gene clusters that likely encode pathways for organohalogen compound production in actinomycetes other than streptomycetes.