Discovery of [NiFe] Hydrogenase Genes in Metagenomic DNA: Cloning and Heterologous Expression in Thiocapsa roseopersicina

Using a metagenomics approach, we have cloned a piece of environmental DNA from the Sargasso Sea that encodes an [NiFe] hydrogenase showing 60% identity to the large subunit and 64% to the small subunit of a Thiocapsa roseopersicina O₂-tolerant [NiFe] hydrogenase. The DNA sequence of the hydrogenase identified by the metagenomic approach was subsequently found to be 99% identical to the hyaA and hyaB genes of an Alteromonas macleodii hydrogenase, indicating that it belongs to the Alteromonas clade. We were able to express our new Alteromonas hydrogenase in T. roseopersicina. Expression was accomplished by coexpressing only two accessory genes, hyaD and hupH, without the need to express any of the hyp accessory genes (hypABCDEF). These results suggest that the native accessory proteins in T. roseopersicina could substitute for the Alteromonas counterparts that are absent in the host to facilitate the assembly of a functional Alteromonas hydrogenase. To further compare the complex assembly machineries of these two [NiFe] hydrogenases, we performed complementation experiments by introducing the new Alteromonas hyaD gene into the T. roseopersicina hynD mutant. Interestingly, Alteromonas endopeptidase HyaD could complement T. roseopersicina HynD to cleave endoproteolytically the C-terminal end of the T. roseopersicina HynL hydrogenase large subunit and activate the enzyme. This study refines our knowledge on the selectivity and pleiotropy of the elements of the [NiFe] hydrogenase assembly machineries. It also provides a model for functionally analyzing novel enzymes from environmental microbes in a culture-independent manner.Hydrogen is a promising energy carrier for the future (10). Photosynthetic microbes such as cyanobacteria have attracted considerable attention, because they can split water photolytically to produce H₂. However, one major drawback of the processes is that their H₂-evolving hydrogenases are extremely sensitive to O₂, which is an inherent by-product of oxygenic photosynthesis. Thus, transfer of O₂-tolerant [NiFe] hydrogenases into cyanobacteria might be one approach to overcome this O₂ sensitivity issue. A small number of O₂-tolerant hydrogenases has been identified (9, 21, 47). However, they tend to favor H₂ uptake over evolution. Searching for novel O₂-tolerant [NiFe] hydrogenases from environmental microbes therefore becomes an important part of the effort to construct such biophotolytic systems.The oceans harbor an abundance of microorganisms with H₂ production capability. Traditionally, new hydrogenases have been screened only from culturable organisms. However, since only a few microbes can be cultured (14), many of them have not been identified, and their functions remain unknown. Metagenomics is a rapidly growing field, which allows us to obtain information about uncultured microbes and to understand the true diversity of microbes in their natural environments. Metagenomics analysis provides a completely new approach for identifying novel [NiFe] hydrogenases from the oceans in a culture-independent manner. The Global Ocean Sampling (GOS) expedition has produced the largest metagenomic data set to date, providing a rich catalog of proteins and protein families, including those enzymes involved in hydrogen metabolism (45, 52, 56-58). Putative novel [NiFe] hydrogenase enzymes that were identified from marine microbial metagenomic data in these expeditions can be examined to find potentially important new hydrogenases. Because source organisms for metagenomic sequences are not typically known, these hydrogenases have to be heterologously expressed in culturable foreign hosts for protein and functional analyses.Unlike most proteins, hydrogenases have a complex architecture and must be assembled and matured through a multiple-step process (7, 11). Hydrogenases are divided into three distinct groups based on their metal contents (54): Fe-S cluster-free hydrogenases (22, 23, 48), [FeFe] hydrogenases (1, 12, 25), and [NiFe] hydrogenases (2, 3, 55). [NiFe] hydrogenases are heterodimers composed of a large subunit and a small subunit, and their NiFe catalytic centers are located in the large subunits (2, 15, 19, 40). A whole set of accessory proteins are required to properly assemble the catalytic centers (7). The accessory protein HypE first interacts with HypF to form a HypF-HypE complex, and the carbamyl group linked to HypF is then dehydrated by HypE in the presence of ATP to release the CN group that is transferred to iron through a HypC-HypD-HypE complex (6). The origin of the CO ligand that is also bound to the iron is not clear, and possibly it comes from formate, formyl-tetrahydrofolate, or acetate. The liganded Fe atom is inserted into the immature large subunit, in which HypC proteins function as chaperones to facilitate the metal insertion (5, 34, 36). Ni is delivered to the catalytic center by the zinc-metalloenzyme HypA that interacts with HypB, a nickel-binding and GTP-hydrolyzing protein. The final step in the maturation process is endoproteolytic cleavage. Once the nickel is transferred to the active site, the endopeptidase, such as HyaD or HynD, cleaves the C-terminal end of the large subunit (33, 43), which triggers a conformational change of the protein so that the Ni-Fe catalytic center can be internalized.Heterologous expression of functional [NiFe] hydrogenases has been demonstrated in several studies (4, 18, 31, 39, 44, 50), suggesting that it could be a feasible approach to express novel hydrogenases from the environment for functional analysis. In this study, we sought to prove the concept that metagenomically derived environmental DNA can give rise to a functional [NiFe] hydrogenase through expression in a foreign host and that novel [NiFe] hydrogenases from environmental microbes can be studied in a culture-independent manner. We cloned environmental DNA that harbors the genes of a putative novel hydrogenase that shows strong homology to a known O₂-tolerant hydrogenase, HynSL, from the phototrophic purple sulfur bacterium Thiocapsa roseopersicina (21, 28, 41, 59). We heterologously expressed the two structural genes (hyaA and hyaB) and two accessory genes (hupH and hyaD) of this novel environmental hydrogenase in T. roseopersicina, a foreign host that may already have the necessary machinery required to process the environmental hydrogenase since it carries the homologous hydrogenase HynSL. We analyzed the new hydrogenase protein and its functions. In addition, we compared the maturation mechanisms between the two homolog hydrogenases by performing complementation experiments.     

Identification of a Novel Class of Membrane-Bound [NiFe]-Hydrogenases in Thermococcus onnurineus NA1 by In Silico Analysis

In silico analysis of group 4 [NiFe]-hydrogenases from a hyperthermophilic archaeon, Thermococcus onnurineus NA1, revealed a novel tripartite gene cluster consisting of dehydrogenase-hydrogenase-cation/proton antiporter subunits, which may be classified as the new subgroup 4b of [NiFe]-hydrogenases-based on sequence motifs.Hydrogenases are the key enzymes involved in the metabolism of H₂, catalyzing the following chemical reaction: 2H⁺ + 2e⁻ ↔ H₂. Hydrogenases can be classified into [NiFe]-hydrogenases, [FeFe]-hydrogenases, and [Fe]-hydrogenases, based on their distinctive functional core containing the catalytic metal center (11, 17).The genomic analysis of Thermococcus onnurineus NA1, a hyperthermophilic archaeon isolated from a deep-sea hydrothermal vent area, revealed the presence of several distinct gene clusters encoding seven [NiFe]-hydrogenases and one homolog similar to Mbx (membrane-bound oxidoreductase) from Pyrococcus furiosus (1, 6, 8, 12). According to the classification system of hydrogenases by Vignais et al. (17), three hydrogenases (one F₄₂₀-reducing and two NADP-reducing hydrogenases) belong to group 3 [NiFe]-hydrogenases, and four hydrogenases belong to group 4 [NiFe]-hydrogenases. The group 4 hydrogenases are widely distributed among bacteria and archaea (17), with Hyc and Hyf (hydrogenase 3 and 4, respectively) from Escherichia coli (19), Coo (CO-induced hydrogenase) from Rhodospirillum rubrum (4), Ech (energy-converting hydrogenase) from Methanosarcina barkeri (7), and Mbh (membrane-bound hydrogenase) from P. furiosus (6, 10, 12) being relatively well-characterized hydrogenases in this group. One of the four group 4 hydrogenases from T. onnurineus NA1 was found to be similar in sequence to that of P. furiosus Mbh (10).     

Hitherto Unknown [Fe-Fe]-Hydrogenase Gene Diversity in Anaerobes and Anoxic Enrichments from a Moderately Acidic Fen

Newly designed primers for [Fe-Fe]-hydrogenases indicated that (i) fermenters, acetogens, and undefined species in a fen harbor hitherto unknown hydrogenases and (ii) Clostridium- and Thermosinus-related primary fermenters, as well as secondary fermenters related to sulfate or iron reducers might be responsible for hydrogen production in the fen. Comparative analysis of [Fe-Fe]-hydrogenase and 16S rRNA gene-based phylogenies indicated the presence of homologous multiple hydrogenases per organism and inconsistencies between 16S rRNA gene- and [Fe-Fe]-hydrogenase-based phylogenies, necessitating appropriate qualification of [Fe-Fe]-hydrogenase gene data for diversity analyses.Molecular hydrogen (H₂) is important in intermediary ecosystem metabolism (i.e., processes that link input to output) in wetlands (7, 11, 12, 33) and other anoxic habitats like sewage sludges (34) and the intestinal tracts of animals (9, 37). H₂-producing fermenters have been postulated to form trophic links to H₂-consuming methanogens, acetogens (i.e., organisms capable of using the acetyl-coenzyme A [CoA] pathway for acetate synthesis) (7), Fe(III) reducers (17), and sulfate reducers in a well-studied moderately acidic fen in Germany (11, 12, 16, 18, 22, 33). 16S rRNA gene analysis revealed the presence of Clostridium spp. and Syntrophobacter spp., which represent possible primary and secondary fermenters, as well as H₂ producers in this fen (11, 18, 33). However, H₂-producing bacteria are polyphyletic (30, 31, 29). Thus, a structural marker gene is required to target this functional group by molecular methods. [Fe-Fe]-hydrogenases catalyze H₂ production in fermenters (19, 25, 29, 30, 31), and genes encoding [Fe-Fe]-hydrogenases represent such a marker gene. The objectives of this study were to (i) develop primers specific for highly diverse [Fe-Fe]-hydrogenase genes, (ii) analyze [Fe-Fe]-hydrogenase genes in pure cultures of fermenters, acetogens, and a sulfate reducer, (iii) assess [Fe-Fe]-hydrogenase gene diversity in H₂-producing fen soil enrichments, and (iv) evaluate the limitations of the amplified [Fe-Fe]-hydrogenase fragment as a phylogenetic marker.     

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).     

Role of the Hya Hydrogenase in Recycling of Anaerobically Produced H2 in Salmonella enterica Serovar Typhimurium

Double and triple uptake-type hydrogenase mutants were used to determine which hydrogenase recycles fermentatively produced hydrogen. The Δhyb Δhya and Δhyd Δhya double mutants evolved H₂ at rates similar to that of the triple mutant strain, so Hya alone oxidizes the bulk of H₂ produced during fermentation. When only Hya was present, no hydrogen production was observed in nutrient-limited medium. H₂ uptake assays showed that Hya can oxidize both exogenously added H₂ and formate hydrogen lyase-evolved H₂ anaerobically. Even after anaerobic growth, all three uptake-type hydrogenases could function in the presence of oxygen, including using O₂ as a terminal acceptor.Due to the anticipated scarcity of fossil fuels, there has been a surge of interest in H₂ production for alternative energy means. Numerous studies have attempted to engineer H₂-producing organisms, such as photosynthetic bacteria, cyanobacteria, and Escherichia coli, to produce maximal amounts of H₂ while minimizing the H₂-oxidizing capability of the organism (4, 5, 9, 10, 19). Hydrogenase expression and activity are controlled by multiple regulatory pathways and respond to fluctuations in pH, oxygen levels, and availability of metabolites and metal cofactors (17). In addition, the presence of hydrogen uptake hydrogenases decreases the net H₂ yield even under conditions that favor H₂ production. It is therefore important to understand the interactions between H₂-oxidizing enzymes (i.e., respiratory hydrogenases) and H₂-producing enzymes.Gene sequence analysis has revealed that many enteric bacteria contain the genes necessary for hydrogen production and oxidation. The E. coli hydrogenases have been studied extensively, while Salmonella enterica serovar Typhimurium hydrogenases have been studied to a lesser extent. Both E. coli and Salmonella serovar Typhimurium contain the hydrogen-oxidizing hydrogenases Hya and Hyb. Salmonella serovar Typhimurium also contains Hyd, which is another hydrogen-oxidizing hydrogenase (2, 13, 15). Hyc and Hyf are hydrogen-evolving hydrogenases that are present in both E. coli and Salmonella serovar Typhimurium, although it is unknown whether Hyf is functional (1).The Salmonella serovar Typhimurium hydrogenases are important for cellular metabolism. Hyc produces H₂ in order to remove excess reductant generated during mixed-acid fermentation. Hyc and formate dehydrogenase constitute the formate hydrogen lyase (FHL) complex (16), which oxidizes formate to produce CO₂ and H₂ (12). The hyb genes in E. coli and Salmonella serovar Typhimurium are expressed at high levels under anaerobic respiration conditions, and Hyb probably contributes to energy conservation (11, 15, 20). Hyb oxidizes H₂ and generates electrons, which are passed through the electron transport chain to terminal acceptors such as fumarate. The protons generated contribute to the proton-motive force. The role of Hya is not as well characterized. Hya may be used to recycle Hyc-produced H₂, since the hya operon is expressed at high levels during fermentative growth, or it may play a role in acid stress resistance (6, 14, 20, 21). The hyb genes are expressed at high levels under aerobic conditions in Salmonella serovar Typhimurium, and Hyb may couple H₂ oxidation to O₂ reduction (20).Redwood et al. recently examined the roles of uptake-type hydrogenases on net hydrogen production in E. coli (10). Cells were pregrown aerobically or anaerobically with formate and then allowed to ferment in anaerobic bottles. H₂ gas was collected, and other fermentation products were measured. They found that H₂ production increased by 37% in an hya hyb double mutant (compared to that in the wild type) that was grown overnight aerobically with formate. This increase in production was associated with the loss of hyb and not hya. Therefore, in E. coli, Hyb may be responsible for recycling fermentatively produced H₂.In this study, we measured the effect of uptake-type hydrogenase mutations on H₂ production in Salmonella serovar Typhimurium. We found that the majority of H₂-recycling activity in fermenting cells was dependent on the presence of hya, and having only Hya was sufficient to prevent any detectable H₂ evolution. These results demonstrate yet another difference between H₂ metabolism in E. coli and H₂ metabolism in Salmonella serovar Typhimurium.     

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.     

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.     

Population Structure of the Lyme Borreliosis Spirochete Borrelia burgdorferi in the Western Black-Legged Tick (Ixodes pacificus) in Northern California

Factors potentially contributing to the lower incidence of Lyme borreliosis (LB) in the far-western than in the northeastern United States include tick host-seeking behavior resulting in fewer human tick encounters, lower densities of Borrelia burgdorferi-infected vector ticks in peridomestic environments, and genetic variation among B. burgdorferi spirochetes to which humans are exposed. We determined the population structure of B. burgdorferi in over 200 infected nymphs of the primary bridging vector to humans, Ixodes pacificus, collected in Mendocino County, CA. This was accomplished by sequence typing the spirochete lipoprotein ospC and the 16S-23S rRNA intergenic spacer (IGS). Thirteen ospC alleles belonging to 12 genotypes were found in California, and the two most abundant, ospC genotypes H3 and E3, have not been detected in ticks in the Northeast. The most prevalent ospC and IGS biallelic profile in the population, found in about 22% of ticks, was a new B. burgdorferi strain defined by ospC genotype H3. Eight of the most common ospC genotypes in the northeastern United States, including genotypes I and K that are associated with disseminated human infections, were absent in Mendocino County nymphs. ospC H3 was associated with hardwood-dominated habitats where western gray squirrels, the reservoir host, are commonly infected with LB spirochetes. The differences in B. burgdorferi population structure in California ticks compared to the Northeast emphasize the need for a greater understanding of the genetic diversity of spirochetes infecting California LB patients.In the United States, Lyme borreliosis (LB) is the most commonly reported vector-borne illness and is caused by infection with the spirochete Borrelia burgdorferi (3, 9, 52). The signs and symptoms of LB can include a rash, erythema migrans, fever, fatigue, arthritis, carditis, and neurological manifestations (50, 51). The black-legged tick, Ixodes scapularis, and the western black-legged tick, Ixodes pacificus, are the primary vectors of B. burgdorferi to humans in the United States, with the former in the northeastern and north-central parts of the country and the latter in the Far West (9, 10). These ticks perpetuate enzootic transmission cycles together with a vertebrate reservoir host such as the white-footed mouse, Peromyscus leucopus, in the Northeast and Midwest (24, 35), or the western gray squirrel, Sciurus griseus, in California (31, 46).B. burgdorferi is a spirochete species with a largely clonal population structure (14, 16) comprising several different strains or lineages (8). The polymorphic ospC gene of B. burgdorferi encodes a surface lipoprotein that increases expression within the tick during blood feeding (47) and is required for initial infection of mammalian hosts (25, 55). To date, approximately 20 North American ospC genotypes have been described (40, 45, 49, 56). At least four, and possibly up to nine, of these genotypes are associated with B. burgdorferi invasiveness in humans (1, 15, 17, 49, 57). Restriction fragment length polymorphism (RFLP) and, subsequently, sequence analysis of the 16S-23S rRNA intergenic spacer (IGS) are used as molecular typing tools to investigate genotypic variation in B. burgdorferi (2, 36, 38, 44, 44, 57). The locus maintains a high level of variation between related species, and this variation reflects the heterogeneity found at the genomic level of the organism (37). The IGS and ospC loci appear to be linked (2, 8, 26, 45, 57), but the studies to date have not been representative of the full range of diversity of B. burgdorferi in North America.Previous studies in the northeastern and midwestern United States have utilized IGS and ospC genotyping to elucidate B. burgdorferi evolution, host strain specificity, vector-reservoir associations, and disease risk to humans. In California, only six ospC and five IGS genotypes have been described heretofore in samples from LB patients or I. pacificus ticks (40, 49, 56) compared to approximately 20 ospC and IGS genotypes identified in ticks, vertebrate hosts, or humans from the Northeast and Midwest (8, 40, 45, 49, 56). Here, we employ sequence analysis of both the ospC gene and IGS region to describe the population structure of B. burgdorferi in more than 200 infected I. pacificus nymphs from Mendocino County, CA, where the incidence of LB is among the highest in the state (11). Further, we compare the Mendocino County spirochete population to populations found in the Northeast.     

Roles of Minor Pilin Subunits Spy0125 and Spy0130 in the Serotype M1 Streptococcus pyogenes Strain SF370

Adhesive pili on the surface of the serotype M1 Streptococcus pyogenes strain SF370 are composed of a major backbone subunit (Spy0128) and two minor subunits (Spy0125 and Spy0130), joined covalently by a pilin polymerase (Spy0129). Previous studies using recombinant proteins showed that both minor subunits bind to human pharyngeal (Detroit) cells (A. G. Manetti et al., Mol. Microbiol. 64:968-983, 2007), suggesting both may act as pilus-presented adhesins. While confirming these binding properties, studies described here indicate that Spy0125 is the pilus-presented adhesin and that Spy0130 has a distinct role as a wall linker. Pili were localized predominantly to cell wall fractions of the wild-type S. pyogenes parent strain and a spy0125 deletion mutant. In contrast, they were found almost exclusively in culture supernatants in both spy0130 and srtA deletion mutants, indicating that the housekeeping sortase (SrtA) attaches pili to the cell wall by using Spy0130 as a linker protein. Adhesion assays with antisera specific for individual subunits showed that only anti-rSpy0125 serum inhibited adhesion of wild-type S. pyogenes to human keratinocytes and tonsil epithelium to a significant extent. Spy0125 was localized to the tip of pili, based on a combination of mutant analysis and liquid chromatography-tandem mass spectrometry analysis of purified pili. Assays comparing parent and mutant strains confirmed its role as the adhesin. Unexpectedly, apparent spontaneous cleavage of a labile, proline-rich (8 of 14 residues) sequence separating the N-terminal ∼1/3 and C-terminal ∼2/3 of Spy0125 leads to loss of the N-terminal region, but analysis of internal spy0125 deletion mutants confirmed that this has no significant effect on adhesion.The group A Streptococcus (S. pyogenes) is an exclusively human pathogen that commonly colonizes either the pharynx or skin, where local spread can give rise to various inflammatory conditions such as pharyngitis, tonsillitis, sinusitis, or erysipelas. Although often mild and self-limiting, GAS infections are occasionally very severe and sometimes lead to life-threatening diseases, such as necrotizing fasciitis or streptococcal toxic shock syndrome. A wide variety of cell surface components and extracellular products have been shown or suggested to play important roles in S. pyogenes virulence, including cell surface pili (1, 6, 32). Pili expressed by the serotype M1 S. pyogenes strain SF370 mediate specific adhesion to intact human tonsil epithelia and to primary human keratinocytes, as well as cultured keratinocyte-derived HaCaT cells, but not to Hep-2 or A549 cells (1). They also contribute to adhesion to a human pharyngeal cell line (Detroit cells) and to biofilm formation (29).Over the past 5 years, pili have been discovered on an increasing number of important Gram-positive bacterial pathogens, including Bacillus cereus (4), Bacillus anthracis (4, 5), Corynebacterium diphtheriae (13, 14, 19, 26, 27, 44, 46, 47), Streptococcus agalactiae (7, 23, 38), and Streptococcus pneumoniae (2, 3, 24, 25, 34), as well as S. pyogenes (1, 29, 32). All these species produce pili that are composed of a single major subunit plus either one or two minor subunits. During assembly, the individual subunits are covalently linked to each other via intermolecular isopeptide bonds, catalyzed by specialized membrane-associated transpeptidases that may be described as pilin polymerases (4, 7, 25, 41, 44, 46). These are related to the classical housekeeping sortase (usually, but not always, designated SrtA) that is responsible for anchoring many proteins to Gram-positive bacterial cell walls (30, 31, 33). The C-terminal ends of sortase target proteins include a cell wall sorting (CWS) motif consisting, in most cases, of Leu-Pro-X-Thr-Gly (LPXTG, where X can be any amino acid) (11, 40). Sortases cleave this substrate between the Thr and Gly residues and produce an intermolecular isopeptide bond linking the Thr to a free amino group provided by a specific target. In attaching proteins to the cell wall, the target amino group is provided by the lipid II peptidoglycan precursor (30, 36, 40). In joining pilus subunits, the target is the ɛ-amino group in the side chain of a specific Lys residue in the second subunit (14, 18, 19). Current models of pilus biogenesis envisage repeated transpeptidation reactions adding additional subunits to the base of the growing pilus, until the terminal subunit is eventually linked covalently via an intermolecular isopeptide bond to the cell wall (28, 41, 45).The major subunit (sometimes called the backbone or shaft subunit) extends along the length of the pilus and appears to play a structural role, while minor subunits have been detected either at the tip, the base, and/or at occasional intervals along the shaft, depending on the species (4, 23, 24, 32, 47). In S. pneumoniae and S. agalactiae one of the minor subunits acts as an adhesin, while the second appears to act as a linker between the base of the assembled pilus and the cell wall (7, 15, 22, 34, 35). It was originally suggested that both minor subunits of C. diphtheriae pili could act as adhesins (27). However, recent data showed one of these has a wall linker role (26, 44) and may therefore not function as an adhesin.S. pyogenes strain SF370 pili are composed of a major (backbone) subunit, termed Spy0128, plus two minor subunits, called Spy0125 and Spy0130 (1, 32). All three are required for efficient adhesion to target cells (1). Studies employing purified recombinant proteins have shown that both of the minor subunits, but not the major subunit, bind to Detroit cells (29), suggesting both might act as pilus-presented adhesins. Here we report studies employing a combination of recombinant proteins, specific antisera, and allelic replacement mutants which show that only Spy0125 is the pilus-presented adhesin and that Spy0130 has a distinct role in linking pili to the cell wall.