四种红树植物根际土壤微生物Ⅰ型和Ⅱ型PKS基因的检测与多样性分析

[目的]探究红树林土壤中聚酮合酶(Polyketide synthase,PKS)基因的多样性和新颖性.[方法]用Ⅰ型和Ⅱ型PKS基因酮基合成酶(Ketosynthase,KS)域的简并引物对海南清澜港红树林海莲、黄槿、银叶、老鼠簕4种红树根际土壤样品中DNA进行PCR扩增,之后利用PCR-限制性酶切片段多样性(PCR-RFLP)和测序分析法对Ⅰ型和Ⅱ型PKS基因的多样性进行探讨.[结果]对得到的72条Ⅰ型PKS基因的酮基合成酶(Ketosynthase,KS)域DNA序列进行PCR-RFLP分析,共得到51个可操作分类单元(Operational taxonomic unit,OTUs),其中37个OTUs为单克隆产生,没有明显的优势OTU.选取了26个代表不同OTU的克隆进行测序分析,这些序列与GenBank中已知序列的最大相似率均未超过85％. KS域氨基酸序列的系统发育分析显示,所得KS域来源广泛,包括蓝细菌门(Cyanobacteria)、变形杆菌门(Proteobacteria)、厚壁菌门(Firmicutes)、放线菌门(Actinobacteria)和一些未可培养细菌;对55条PKSⅡ基因KS域DNA序列的PCR-RFLP分析后共得到25个OTUs,有两个明显的优势OTUs,代表的克隆子数所占比例超过10％.[结论]PCR-RFLP分析表明红树林根际土壤中存在着丰富多样的Ⅰ型和Ⅱ型PKS基因,且前者多样性更高;低的序列相似度表明所获得的PKSⅠ基因KS域序列独特;系统发育分析表明得到的PKSⅠ基因来源广泛.     

Metagenomic Analysis Reveals Diverse Polyketide Synthase Gene Clusters in Microorganisms Associated with the Marine Sponge Discodermia dissoluta

Sponge-associated bacteria are thought to produce many novel bioactive compounds, including polyketides. PCR amplification of ketosynthase domains of type I modular polyketide synthases (PKS) from the microbial community of the marine sponge Discodermia dissoluta revealed great diversity and a novel group of sponge-specific PKS ketosynthase domains. Metagenomic libraries totaling more than four gigabases of bacterial genomes associated with this sponge were screened for type I modular PKS gene clusters. More than 90% of the clones in total sponge DNA libraries represented bacterial DNA inserts, and 0.7% harbored PKS genes. The majority of the PKS hybridizing clones carried small PKS clusters of one to three modules, although some clones encoded large multimodular PKSs (more than five modules). The most abundant large modular PKS appeared to be encoded by a bacterial symbiont that made up <1% of the sponge community. Sequencing of this PKS revealed 14 modules that, if expressed and active, is predicted to produce a multimethyl-branched fatty acid reminiscent of mycobacterial lipid components. Metagenomic libraries made from fractions enriched for unicellular or filamentous bacteria differed significantly, with the latter containing numerous nonribosomal peptide synthetase (NRPS) and mixed NRPS-PKS gene clusters. The filamentous bacterial community of D. dissoluta consists mainly of Entotheonella spp., an unculturable sponge-specific taxon previously implicated in the biosynthesis of bioactive peptides.     

Polyketide synthase pathways identified from a metagenomic library are derived from soil Acidobacteria

Polyketides are structurally diverse secondary metabolites, many of which have antibiotic or anticancer activity. Type I modular polyketide synthase (PKS) genes are typically large and encode repeating enzymatic domains that elongate and modify the nascent polyketide chain. A fosmid metagenomic library constructed from an agricultural soil was arrayed and the macroarray was screened for the presence of conserved ketosynthase [β-ketoacyl synthase (KS)] domains, enzymatic domains present in PKSs. Thirty-four clones containing KS domains were identified by Southern hybridization. Many of the KS domains contained within metagenomic clones shared significant similarity to PKS or nonribosomal peptide synthesis genes from members of the Cyanobacteria or the Proteobacteria phyla. However, analysis of complete clone insert sequences indicated that the blast analysis for KS domains did not reflect the true phylogenetic origin of many of these metagenomic clones that had a %G+C content and significant sequence similarity to genes from members of the phylum Acidobacteria. This conclusion of an Acidobacteria origin for several clones was further supported by evidence that cultured soil Acidobacteria from different subdivisions have genetic loci closely related to PKS domains contained within metagenomic clones, suggesting that Acidobacteria may be a source of novel polyketides. This study also demonstrates the utility of combining data from culture-dependent and -independent investigations in expanding our collective knowledge of microbial genomic diversity.     

Identification of unique type II polyketide synthase genes in soil

Many bacteria, particularly actinomycetes, are known to produce secondary metabolites synthesized by polyketide synthases (PKS). Bacterial polyketides are a particularly rich source of bioactive molecules, many of which are of potential pharmaceutical relevance. To directly access PKS gene diversity from soil, we developed degenerate PCR primers for actinomycete type II KS(alpha) (ketosynthase) genes. Twenty-one soil samples were collected from diverse sources in New Jersey, and their bacterial communities were compared by terminal restriction fragment length polymorphism (TRFLP) analysis of PCR products generated using bacterial 16S rRNA gene primers (27F and 1525R) as well as an actinomycete-specific forward primer. The distribution of actinomycetes was highly variable but correlated with the overall bacterial species composition as determined by TRFLP. Two samples were identified to contain a particularly rich and unique actinomycete community based on their TRFLP patterns. The same samples also contained the greatest diversity of KS(alpha) genes as determined by TRFLP analysis of KS(alpha) PCR products. KS(alpha) PCR products from these and three additional samples with interesting TRFLP pattern were cloned, and seven novel clades of KS(alpha) genes were identified. Greatest sequence diversity was observed in a sample containing a moderate number of peaks in its KS(alpha) TRFLP. The nucleotide sequences were between 74 and 81% identical to known sequences in GenBank. One cluster of sequences was most similar to the KS(alpha) involved in ardacin (glycopeptide antibiotic) production by Kibdelosporangium aridum. The remaining sequences showed greatest similarity to the KS(alpha) genes in pathways producing the angucycline-derived antibiotics simocyclinone, pradimicin, and jasomycin.     

Characterization of two novel non-reducing polyketide synthase genes from the lichen-forming fungus Hypogymnia physodes

Lichens are known to produce a variety of secondary metabolites including polyketides, which have valuable biological activities. Some polyketides are produced solely by lichens. The biosynthesis of these compounds is primarily governed by iterative type I polyketide synthases. Hypogymnia physodes synthesize polyketides such as physodic, physodalic and hydroxyphysodic acid and atranorin, which are non-reducing polyketides. Two novel non-reducing polyketide synthase (PKS) genes were isolated from a fosmid genomic library of a mycobiont of H. physodes using a 409bp fragment corresponding to part of the reductase (R) domain as a probe. H. physodes PKS1 (Hyopks1) and PKS2 (Hypopks2) contain keto synthase (KS), acyl transferase (AT), acyl carrier protein (ACP), methyl transferase (ME) and R domains. Classification based on phylogeny analysis using the translated KS and AT domains demonstrated that Hypopks1 and Hypopks2 are members of the fungal non-reducing PKSs clade III. This is the first report of non-reducing PKSs containing the R domain-mediated release mechanisms in lichens, which are also rare fungal type I PKS in non-lichenized filamentous fungi.     

A computationally simplistic poly-phasic approach to explore microbial communities from the Yucatan aquifer as a potential sources of novel natural products

The need for new antibiotics has sparked a search for the microbes that might potentially produce them. Current sequencing technologies allow us to explore the biotechnological potential of microbial communities in diverse environments without the need for cultivation, benefitting natural product discovery in diverse ways. A relatively recent method to search for the possible production of novel compounds includes studying the diverse genes belonging to polyketide synthase pathways (PKS), as these complex enzymes are an important source of novel therapeutics. In order to explore the biotechnological potential of the microbial community from the largest underground aquifer in the world located in the Yucatan, we used a polyphasic approach in which a simple, non-computationally intensive method was coupled with direct amplification of environmental DNA to assess the diversity and novelty of PKS type I ketosynthase (KS) domains. Our results suggest that the bioinformatic method proposed can indeed be used to assess the novelty of KS enzymes; nevertheless, this in silico study did not identify some of the KS diversity due to primer bias and stringency criteria outlined by the metagenomics pipeline. Therefore, additionally implementing a method involving the direct cloning of KS domains enhanced our results. Compared to other freshwater environments, the aquifer was characterized by considerably less diversity in relation to known ketosynthase domains; however, the metagenome included a family of KS type I domains phylogenetically related, but not identical, to those found in the curamycin pathway, as well as an outstanding number of thiolases. Over all, this first look into the microbial community found in this large Yucatan aquifer and other fresh water free living microbial communities highlights the potential of these previously overlooked environments as a source of novel natural products.     

Metagenomic analysis reveals diverse polyketide synthase gene clusters in microorganisms associated with the marine sponge Discodermia dissoluta

Sponge-associated bacteria are thought to produce many novel bioactive compounds, including polyketides. PCR amplification of ketosynthase domains of type I modular polyketide synthases (PKS) from the microbial community of the marine sponge Discodermia dissoluta revealed great diversity and a novel group of sponge-specific PKS ketosynthase domains. Metagenomic libraries totaling more than four gigabases of bacterial genomes associated with this sponge were screened for type I modular PKS gene clusters. More than 90% of the clones in total sponge DNA libraries represented bacterial DNA inserts, and 0.7% harbored PKS genes. The majority of the PKS hybridizing clones carried small PKS clusters of one to three modules, although some clones encoded large multimodular PKSs (more than five modules). The most abundant large modular PKS appeared to be encoded by a bacterial symbiont that made up < 1% of the sponge community. Sequencing of this PKS revealed 14 modules that, if expressed and active, is predicted to produce a multimethyl-branched fatty acid reminiscent of mycobacterial lipid components. Metagenomic libraries made from fractions enriched for unicellular or filamentous bacteria differed significantly, with the latter containing numerous nonribosomal peptide synthetase (NRPS) and mixed NRPS-PKS gene clusters. The filamentous bacterial community of D. dissoluta consists mainly of Entotheonella spp., an unculturable sponge-specific taxon previously implicated in the biosynthesis of bioactive peptides.     

PCR Detection of Type I Polyketide Synthase Genes in Myxobacteria

The diversity of type I modular polyketide synthase (PKS) was explored by PCR amplification of DNA encoding ketosynthase and acyltransferase domains in myxobacteria. The sequencing of the amplicons revealed that many PKS genes were distantly related to the published sequences. Thus, myxobacteria may be excellent resources for novel and diverse polyketides.     

Polyketide synthases of bacterial symbionts in sponges – Evolution-based applications in natural products research

Marine sponges are an unusually rich source of bioactive natural products with clinical potential. They also often harbor rich communities of symbiotic bacteria that have often been suspected as the true producers of sponge-derived compounds. To date, these bacteria can in most cases not be cultivated, but culture-independent methods, such as isolating and analyzing biosynthetic gene clusters using metagenomic strategies, have recently provided first insights into their chemical potential. This review summarizes recent work of our laboratory on the study of polyketide synthases (PKSs). These studies revealed two evolutionarily distinct, unusual PKS types that are commonly found in sponge metagenomes and were shown to be of bacterial origin. One, the sup PKS, dominates sponge metagenomic DNA libraries, occurs widespread in bacteriosponges and is to date exclusively known from such animals. Data suggest that it is a type of synthase that generates methyl-branched fatty acids, which are commonly present in sponges. The other PKS type, termed trans-acyltransferase (AT) PKS, is responsible for the biosynthesis of complex, bioactive polyketides, such as the onnamides, and also occurs in free-living bacteria. The diversity of PKS genes present in a single sponge metagenome can be enormous. However, the phylogenetic approaches outlined in this review can provide valuable insights into the PKS function and structures of polyketides and can assist in the targeted isolation of gene clusters.     

Metabolic diversity in myxobacteria: identification of the myxalamid and the stigmatellin biosynthetic gene cluster of Stigmatella aurantiaca Sg a15 and a combined polyketide-(poly)peptide gene cluster from the epothilone producing strain Sorangium cellulosum So ce90.

Myxobacterial strains producing polyketides (PKs) assumed to be biosynthesized by a type I polyketide synthase (PKS) were analysed. Myxobacteria also produce a variety of polypeptides (PP) and PKs with incorporated amino acids ('mixed PK-PP'). In order to be able to identify the biosynthetic gene clusters for these metabolites a PCR based approach has been developed to clone ketosynthase (KS) domains of PKS genes from these organisms. Conserved regions of peptide synthetases of the non-ribosomal type (NRPS) were also amplified via PCR. KS fragments from Stigmatella aurantiaca Sg a15 were used for chromosomal gene inactivation experiments resulting in a series of mutants including such that were unable to produce stigmatellins and myxalamids. A NRPS fragment and PKS fragments from Sorangium cellulosum So ce90 were used to identify cosmids hybridizing with both types of probes from a genomic library. Both a NRPS and a PKS fragment were cloned and sequenced from a relatively short restriction fragment of one of these cosmids. The method described here should be very useful to clone and identify PKS, NRPS and mixed PKS-NRPS from myxobacteria in general and thereby open opportunities to use the biochemical diversity of these bacteria for genetic engineering and combinatorial biosynthesis.     

Chemical investigation of novel ascomycetes using PCR based screening approaches

Fungi are well known for a wealth of pharmacologically important activities and agrochemical properties. Polyketides that are widely found in fungi, are a large group of secondary metabolites which exhibit diversity in their function and structure. Here we described an investigation of three fungal strains which were prospected for production of polyketides. The aim of this work was to employ the diversity of reducing type I polyketide synthase genes in these fungi using a molecular and bioinformatics approaches as a mini tool. A degenerate primer pair for highly reduced PKSs was newly designed and used together with ketosynthase primers for amplification. One hundred and thirty-eight clones were sequenced. Ten KS domain sequences were isolated, using two primer pairs specific for highly reduced type PKSs. This study revealed four sequences from Emarcea castanopsidicola, four ketosynthase sequences from Gaeumannomyces amomi and two sequences from Leiosphaerella amomi, respectively. Bioinformatic techniques were employed to identify a group of these KS domain sequences. Based on these sequences suggested that rapid screening provided the potential to explore significant PKS structural diversity. Hence chemical investigation had been conducted and exhibited nine compounds. The endophytic fungus L. amomi was cultivated and elucidated linoleic acid, ergosterol and an unidentified sterol in the extracts. Linoleic acid, sitosterol, and p-hydroxybenzoic acid were isolated from the saprobic fungus E. castanopsidicola. We first isolated a new polyketide, stemphol 1-O-β-D-galactopyranoside together with four known metabolites; stemphol, kojic acid, ergosterol, indole-3-carboxylic acid from an ethyl acetate extract of the cultures of G. amomi. Stemphol was classified as a phenolic lipid or resorcinolic lipid, which have biopharmacological, biomedical, and biotechnological importance. However, recent researches have revealed that these molecule types are synthesized by 2′-oxoalkylresorcinolic acid synthase. The prospective KS domain sequences from this study will be used as probes to isolate putative PKS genes. A gene cluster responsible for PK biosynthesis should be confirmed by determination of PK products generated by these enzymes.     

Phylogenetic analysis of type I polyketide synthase and nonribosomal peptide synthetase genes in Antarctic sediment

The modular polyketide synthase (PKS) and nonribosomal peptide synthetase (NRPS) have been found to be involved in natural product synthesis in many microorganisms. Study on their diversities in natural environment may provide important ecological insights, in addition to opportunities for antibacterial drugs development. In this study, the PKS and NRPS gene diversities in two coast sediments near China Zhongshan Station were studied. The phylogenetic analysis of amino acid (AA) sequences indicated that the identified ketosynthase (KS) domains were clustered with those from diverse bacterial groups, including Proteobacteria, Firmicutes, Planctomycetes, Cyanobacteria, Actinobacteria, and some uncultured symbiotic bacteria. One new branch belonging to hybrid PKS/NRPS enzyme complexes and five independent clades were found on the phylogenetic tree. The obtained adenylation (A) domains were mainly clustered within the Cyanobacteria and Proteobacteria group. Most of the identified KS and A domains showed below 80 and 60% identities at the AA level to their closest matches in GenBank, respectively. The diversities of both KS and A domains in natural environmental sample were different from those in sewage-contaminated sample. These results revealed the great diversity and novelty of both PKS and NRPS genes in Antarctic sediment.     

Diversity and evolution of polyketide biosynthesis gene clusters in the Ceratocystidaceae

Polyketides are secondary metabolites with diverse biological activities. Polyketide synthases (PKS) are often encoded from genes clustered in the same genomic region. Functional analyses and genomic studies show that most fungi are capable of producing a repertoire of polyketides. We considered the potential of Ceratocystidaceae for producing polyketides using a comparative genomics approach. Our aims were to identify the putative polyketide biosynthesis gene clusters, to characterize them and predict the types of polyketide compounds they might produce. We used sequences from nineteen species in the genera, Ceratocystis, Endoconidiophora, Davidsoniella, Huntiella, Thielaviopsis and Bretziella, to identify and characterize PKS gene clusters, by employing a range of bioinformatics and phylogenetic tools. We showed that the genomes contained putative clusters containing a non-reducing type I PKS and a type III PKS. Phylogenetic analyses suggested that these genes were already present in the ancestor of the Ceratocystidaceae. By contrast, the various reducing type I PKS-containing clusters identified in these genomes appeared to have distinct evolutionary origins. Although one of the identified clusters potentially allows for the production of melanin, their functional characterization will undoubtedly reveal many novel and important compounds implicated in the biology of the Ceratocystidaceae.     

Investigation of bacteria with polyketide synthase genes and antimicrobial activity isolated from South China Sea sponges

Aims:  To obtain bacteria with PKS (polyketide synthase) genes and antimicrobial activity from sponges.
Methods and Results:  Eighteen bacteria with KS (ketosynthase) genes were identified by polymerase chain reaction (PCR) screening of 98 isolates from South China Sea sponges, Stelletta tenuis , Halichondria rugosa , Dysidea avara and Craniella australiensis . 16S rRNA gene-based B last analysis indicated that 15 isolates belonged to the phylum Firmicutes , among which 14 isolates were closely related to genus Bacillus , and 1 to Staphylococcus lentus . Two isolates were identified as actinomycetes, and one as Alcaligenes sp. in the phylum Proteobacteria . The 18 KS domains belong to trans-AT type I PKS and match PKS of marine bacterial symbionts. The 18 bacteria exhibited broad-spectrum antimicrobial activities against fungi, gram-positive and gram-negative bacteria. A 21·8-kb PKS gene cluster fragment containing five modules was isolated from the Staphylococcus lentus isolate A75 by screening of a fosmid library.
Conclusions:  The PKS gene diversity and different antimicrobial spectra indicate the potential of bacteria associated with South China Sea sponges for diverse polyketide production.
Significance and Impact of the Study:  Combined with bioactivity assay the PKS gene-based approach can be applied to efficient screening of strains of pharmaceutical value and the prediction of related compounds.     

Evolution of ketosynthase domains of polyketide synthase genes in the Cladonia chlorophaea species complex (Cladoniaceae)

Lichen-forming fungi synthesize a diversity of polyketides, but only a few non-reducing polyketide synthase (PKS) genes from a lichen-forming fungus have been linked with a specific polyketide. While it is a challenge to link the large number of PKS paralogs in fungi with specific products, it might be expected that the PKS paralogs from closely related species would be similar because of recent evolutionary divergence. The objectives of this study were to reconstruct a PKS gene phylogeny of the Cladonia chlorophaea species complex based on the ketosynthase domain, a species phylogeny of the complex, and to explore the presence of PKS gene paralogs among members of the species complex. DNA was isolated from 51 individuals of C. chlorophaea and allies to screen for the presence of 13 PKS paralogs. A 128 sequence PKS gene phylogeny using deduced amino acid sequences estimated from the 13 PKS paralogs and sequences subjected to BLASTx comparisons showed losses of each of two PKS domains (reducing and methylation). This research provided insight into the evolution of PKS genes in the C. chlorophaea group, species evolution in the group, and it identified potential directions for further investigation of polyketide synthesis in the C. chlorophaea species complex.     

Diversity of bacteria and polyketide synthase associated with marine sponge Haliclona sp.

The microbial community associated with a marine sponge (Haliclona sp.) collected from Tateyama city, Japan was studied using 16S rRNA gene clone libraries. Two DNA templates were prepared using methods recommended for Gram-positive and Gram-negative bacteria in the Qiagen kit manual. From each DNA template, two 16S rRNA genes were PCR amplified, using the combination of universal bacterial primer 27f and primers 1385r and 1492r, respectively. A total of 347 clones were sequenced and compared with those available in DNA data banks. These sequences were members of ten bacterial phyla. Interestingly, more than 30 % of the clones represent novel sequences. A comparison of these sequences with sequences in a library prepared from DNA extracted from the surrounding water shows minimum DNA contamination. Taxonomically, the highest diversity was detected in the clone library prepared using a combination of primers 27f and 1492r and DNA isolated using the Gram-positive bacteria protocol. The potential of Haliclona sp.-associated bacteria to produce secondary metabolites was studied by cloning and sequencing the polyketide synthase (PKS, type 1) gene using the same DNA samples. Analysis of partial sequences derived from the sponge metagenome revealed 27 unique ketosynthase domains of PKS type I. This study suggests strongly that this Haliclona sp. plays host to diverse novel bacteria with a potential to produce novel polyketides.     

Genome mining reveals the evolutionary origin and biosynthetic potential of basidiomycete polyketide synthases

Numerous polyketides are known from bacteria, plants, and fungi. However, only a few have been isolated from basidiomycetes. Large scale genome sequencing projects now help anticipate the capacity of basidiomycetes to synthesize polyketides. In this study, we identified and annotated 111 type I and three type III polyketide synthase (PKS) genes from 35 sequenced basidiomycete genomes. Phylogenetic analysis of PKS genes suggests that all main types of fungal iterative PKS had already evolved before the Ascomycota and Basidiomycota diverged. A comparison of genomic and metabolomic data shows that the number of polyketide genes exceeds the number of known polyketide structures by far. Exploiting these results to design degenerate PCR primers, we amplified and cloned the complete sequence of armB, a PKS gene from the melleolide producer Armillaria mellea. We expect this study will serve as a guide for future genomic mining projects to discover structurally diverse mushroom-derived polyketides.     

Chimeric 6-methylsalicylic acid synthase with domains of acyl carrier protein and methyltransferase from Pseudallescheria boydii shows novel biosynthetic activity

Polyketides are important secondary metabolites, many of which exhibit potent pharmacological applications. Biosynthesis of polyketides is carried out by a single polyketide synthase (PKS) or multiple PKSs in successive elongations of enzyme-bound intermediates related to fatty acid biosynthesis. The polyketide gene PKS306 from Pseudallescheria boydii NTOU2362 containing domains of ketosynthase (KS), acyltransferase (AT), dehydratase (DH), acyl carrier protein (ACP) and methyltransferase (MT) was cloned in an attempt to produce novel chemical compounds, and this PKS harbouring green fluorescent protein (GFP) was expressed in Saccharomyces cerevisiae. Although fluorescence of GFP and fusion protein analysed by anti-GFP antibody were observed, no novel compound was detected. 6-methylsalicylic acid synthase (6MSAS) was then used as a template and engineered with PKS306 by combinatorial fusion. The chimeric PKS containing domains of KS, AT, DH and ketoreductase (KR) from 6MSAS with ACP and MT from PKS306 demonstrated biosynthesis of a novel compound. The compound was identified with a deduced chemical formula of C₇H₁₀O₃, and the chemical structure was named as 2-hydroxy-2-(propan-2-yl) cyclobutane-1,3-dione. The novel compound synthesized by the chimeric PKS in this study demonstrates the feasibility of combinatorial fusion of PKS genes to produce novel polyketides.