Isolation and partial characterization of a cryptic polyene gene cluster in Pseudonocardia autotrophica

The polyene antibiotics, a category that includes nystatin, pimaricin, amphotericin, and candicidin, comprise a family of very promising antifungal polyketide compounds and are typically produced by soil actinomycetes. The biosynthetic gene clusters for these polyenes have been previously investigated, revealing the presence of highly similar cytochrome P450 hydroxylase (CYP) genes. Using polyene CYP-specific PCR screening with several actinomycete genomic DNAs, Pseudonocardia autotrophica was determined to contain a unique polyene-specific CYP gene. Genomic DNA library screening using the polyene-specific CYP gene probe identified a positive cosmid clone, which contained a DNA fragment of approximately 34.5 kb. The complete sequencing of this DNA fragment revealed a total of seven complete and two incomplete open reading frames, which were found to be highly similar, but still unique, when compared to previously known polyene biosynthetic genes. These results suggest that the polyene-specific screening approach may constitute an efficient method for the isolation of potentially valuable cryptic polyene biosynthetic gene clusters from various rare actinomycetes.     

多烯大环内酯类抗生素生物合成基因簇中调控因子的研究进展

多烯大环内酯类抗生素具有良好的抗真菌活性,广泛应用于医疗卫生、食品加工和农业生产领域。随着高通量测序技术和生物信息学技术的发展,越来越多的链霉菌抗生素生物合成基因簇被发现和鉴定,调控因子作为生物合成基因簇中的重要组成部分,在庞大复杂的调控网络中起着至关重要的作用。本文总结了链霉菌中重要的调控因子类型,综述了多烯大环内酯类抗生素生物合成基因簇中调控因子的生物学功能、结合位点、作用机制等研究进展,并展望了后续研究工作。     

Isolation of putative polyene-producing actinomycetes strains via PCR-based genome screening for polyene-specific hydroxylase genes

Polyene antibiotics, which include nystatin, pimaricin, amphotericin and candicidin, include a family of very promising antifungal polyketide compounds that are typically produced by soil actinomycetes. The presence of similar cytochrome P450 hydroxylase (CYP) genes in the biosynthetic gene clusters for these polyenes have been previously reported. Using this polyene, more than 200 independently isolated actinomycetes strains were screened by CYP-specific PCR. Four strains were isolated based on the presence of the expected size of the PCR-amplified DNA fragment in the chromosome. The nucleotide sequencing of the PCR-amplified DNA fragments showed that each of the four actinomycetes strains contained a highly homologous polyene-specific CYP gene. Each of the culture extracts from these four strains showed a typical polyene-like high-pressure liquid chromatography (HPLC) chromatogram profile, and strong antifungal activity against Candida albicans. This suggests that the polyene-specific PCR-guided genome screening approach is an efficient method for isolating potentially valuable polyene-producing actinomycetes.     

Identification of functionally clustered nystatin-like biosynthetic genes in a rare actinomycetes, <Emphasis Type="Italic">Pseudonocardia autotrophica</Emphasis>

The polyene antibiotics, including nystatin, pimaricin, amphotericin, and candicidin, comprise a family of very valuable antifungal polyketide compounds, and they are typically produced by soil actinomycetes. Previously, using a polyene cytochrome P450 hydroxylase-specific genome screening strategy, Pseudonocardia autotrophica KCTC9441 was determined to contain genes potentially encoding polyene biosynthesis. Here, sequence information of an approximately 125.7-kb contiguous DNA region in five overlapping cosmids isolated from the P. autotrophica KCTC9441 genomic library revealed a total of 23 open reading frames, which are presumably involved in the biosynthesis of a nystatin-like compound tentatively named NPP. The deduced roles for six multi-modular polyketide synthase (PKS) catalytic domains were found to be highly homologous to those of previously identified nystatin biosynthetic genes. Low NPP productivity suggests that the functionally clustered NPP biosynthetic pathway genes are tightly regulated in P. autotrophica. Disruption of a NPP PKS gene completely abolished both NPP biosynthesis and antifungal activity against Candida albicans, suggesting that polyene-specific genome screening may constitute an efficient method for isolation of potentially valuable previously identified polyene genes and compounds from various rare actinomycetes widespread in nature.     

纳他霉素高产菌株Streptomyces gilvosporeus F607基因组及其生物合成基因簇分析

【背景】纳他霉素(Natamycin)是一种天然、广谱、高效的多烯大环内酯类抗真菌剂,褐黄孢链霉菌(Streptomyces gilvosporeus)是一种重要的纳他霉素产生菌。目前S. gilvosporeus基因组序列分析还未有报道,限制了该菌中纳他霉素及其他次级代谢产物合成及调控的研究。【目的】解析纳他霉素高产菌株S. gilvosporeus F607的基因组序列信息,挖掘其次级代谢产物基因资源,为深入研究该菌株的纳他霉素高产机理及生物合成调控机制奠定基础。【方法】利用相关软件对F607菌株的基因组序列进行基因预测、功能注释、进化分析和共线性分析,并预测次级代谢产物合成基因簇;对纳他霉素生物合成基因簇进行注释分析,比较分析不同菌种中纳他霉素生物合成基因簇的差异;分析预测S.gilvosporeusF607中纳他霉素生物合成途径。【结果】F607菌株基因组总长度为8482298bp,(G+C)mol%为70.95%,分别在COG、GO、KEGG数据库提取到5 062、4 428、5063个基因的注释信息。同时,antiSMASH软件预测得到29个次级代谢产物合成基因簇,其中纳他霉素基因簇与S.natalensis、S. chattanoogensis等菌株的纳他霉素基因簇相似性分别为81%和77%。除2个参与调控的sngT和sgnH基因和9个未知功能的orf基因有差异外,S. gilvosporeus F607基因簇中其他纳他霉素生物合成基因及其排列顺序与已知的纳他霉素基因簇高度一致。【结论】分析了S. gilvosporeus全基因组信息,预测了S. gilvosporeus F607中纳他霉素生物合成的途径,为从基因组层面上解析S. gilvosporeus F607菌株高产纳他霉素的内在原因提供了基础数据,为揭示纳他霉素高产的机理及工业化生产和未来新药的发现奠定了良好的基础。     

An efficient gene transfer system for the pimaricin producer Streptomyces natalensis

Streptomyces natalensis produces the antifungal polyene macrolide pimaricin. Genetic manipulation of its biosynthetic genes has been hampered by the lack of efficient gene transfer systems. We have developed a gene transfer system based on intergeneric conjugation from Escherichia coli. Using this approach, we managed to attain transformation efficiencies of 1 x 10(-4) exconjugants per recipient when using self-replicating vectors such as pHZ1358. The use of integrative vectors such as pSET152 or pSOK804 resulted in significantly lower efficiencies. Site-specific integration or the use of self-replicating plasmids did not affect pimaricin production or the essential functions of S. natalensis. Use of DNA methylation proficient E. coli donor strains resulted in no transformants, indicating the presence of methyl-specific restriction systems in S. natalensis. This methodology will enable easier manipulation of the genes responsible for pimaricin biosynthesis, and could prove valuable for the generation of new designer polyene macrolides with better antifungal activity and pharmacological properties. As an example of the validity of the method, we describe the introduction of Supercos-1-derived cosmid vectors into S. natalensis in order to promote gene replacements by double crossover recombination.     

Analysis and manipulation of amphotericin biosynthetic genes by means of modified phage KC515 transduction techniques

Amphotericin B is a medically important antifungal antibiotic that is produced by Streptomyces nodosus. Genetic manipulation of this organism has led to production of the first amphotericin analogues by engineered biosynthesis. Here, these studies were extended by sequencing the chromosomal regions flanking the amphotericin polyketide synthase genes, and by refining the phage KC515 transduction method for disruption and replacement of S. nodosus genes. A hybrid vector was constructed from KC515 DNA and the Escherichia coli plasmid pACYC177. This vector replicated as a plasmid in E. coli and the purified DNA yielded phage plaques on Streptomyces lividans after polyethylene glycol (PEG)-mediated transfection of protoplasts. The left flank of the amphotericin gene cluster was found to include amphRI, RII, RIII and RIV genes that are similar to regulatory genes in other polyene biosynthetic gene clusters. One of these regulatory genes, amphRI, was found to have a homologue, amphRVI, located in the right flank at a distance of 127 kbp along the chromosome. However, disruption of amphRVI using the hybrid vector had no effect on the yield of amphotericin obtained from cultures grown on production medium. The hybrid vector was also used for precise deletion of the DNA coding for two modules of the AmphC polyketide synthase protein. Analysis by UV spectrophotometry revealed that the deletion mutant produced a novel pentaene, with reduced antifungal activity but apparently greater water-solubility than amphotericin B. This shows the potential for use of the new vector in engineering of this and other biosynthetic pathways in Streptomyces.     

Gene clusters for beta-lactam antibiotics and control of their expression: why have clusters evolved, and from where did they originate?

While beta-lactam compounds were discovered in filamentous fungi, actinomycetes and gram-negative bacteria are also known to produce different types of beta-lactams. All beta-lactam compounds contain a four-membered beta-lactam ring. The structure of their second ring allows these compounds to be classified into penicillins, cephalosporins, clavams, carbapenens or monobactams. Most beta-lactams inhibits bacterial cell wall biosynthesis but others behave as beta-lactamase inhibitors (e.g., clavulanic acid) and even as antifungal agents (e.g., some clavams). Due to the nature of the second ring in beta-lactam molecules, the precursors and biosynthetic pathways of clavams, carbapenems and monobactams differ from those of penicillins and cephalosporins. These last two groups, including cephamycins and cephabacins, are formed from three precursor amino acids that are linked into the alpha-aminoadipyl-L-cysteinyl-D-valine tripeptide. The first two steps of their biosynthetic pathways are common. The intermediates of these pathways, the characteristics of the enzymes involved, the lack of introns in the genes and bioinformatic analysis suggest that all of them should have evolved from an ancestral gene cluster of bacterial origin, which was surely transferred horizontally in the soil from producer to non-producer microorganisms. The receptor strains acquired fragments of the original bacterial cluster and occasionally inserted new genes into the clusters, which once modified, acquired new functions and gave rise to the final compounds that we know. When the order of genes in the Streptomyces genome is analyzed, the antibiotic gene clusters are highlighted as gene islands in the genome. Nonetheless, the assemblage of the ancestral beta-lactam gene cluster remains a matter of speculation.     

The aureolic acid family of antitumor compounds: structure, mode of action, biosynthesis, and novel derivatives

Members of the aureolic acid family are tricyclic polyketides with antitumor activity which are produced by different streptomycete species. These members are glycosylated compounds with two oligosaccharide chains of variable sugar length. They interact with the DNA minor groove in high-GC-content regions in a nonintercalative way and with a requirement for magnesium ions. Mithramycin and chromomycins are the most representative members of the family, mithramycin being used as a chemotherapeutic agent for the treatment of several cancer diseases. For chromomycin and durhamycin A, antiviral activity has also been reported. The biosynthesis gene clusters for mithramycin and chromomycin A₃ have been studied in detail by gene sequencing, insertional inactivation, and gene expression. Most of the biosynthetic intermediates in these pathways have been isolated and characterized. Some of these compounds showed an increase in antitumor activity in comparison with the parent compounds. A common step in the biosynthesis of all members of the family is the formation of the tetracyclic intermediate premithramycinone. Further biosynthetic steps (glycosylation, methylations, acylations) proceed through tetracyclic intermediates which are finally converted into tricyclic compounds by the action of a monooxygenase, a key event for the biological activity. Heterologous expression of biosynthetic genes from other aromatic polyketide pathways in the mithramycin producer (or some mutants) led to the isolation of novel hybrid compounds.Felipe Lombó and Nuria Menéndez have equally contribute to this work.     

The biosynthetic gene cluster for the 26-membered ring polyene macrolide pimaricin. A new polyketide synthase organization encoded by two subclusters separated by functionalization genes

The biosynthetic gene cluster for the 26-membered ring of the polyene macrolide pimaricin extends for about 110 kilobase pairs of contiguous DNA in the genome of Streptomyces natalensis. Two sets of polyketide synthase (PKS) genes are separated by a group of small polyketide-functionalizing genes. Two of the polyketide synthase genes, pimS0 and pimS1, have been fully sequenced and disrupted proving the involvement of each of these genes in pimaricin biosynthesis. The pimS0 gene encodes a relatively small acetate-activating PKS (approximately 193 kDa) that appears to work as a loading protein which "presents" the starter unit to the second PKS subunit. The pimS1 gene encodes a giant multienzyme (approximately 710 kDa) harboring 15 activities responsible for the first four cycles of chain elongation in pimaricin biosynthesis, resulting in formation of the polyene chromophore.     

Biosynthetic gene cluster for the cladoniamides, bis-indoles with a rearranged scaffold

The cladoniamides are bis-indole alkaloids isolated from Streptomyces uncialis, a lichen-associated actinomycete strain. The cladoniamides have an unusual, indenotryptoline structure rarely observed among bis-indole alkaloids. I report here the isolation, sequencing, and annotation of the cladoniamide biosynthetic gene cluster and compare it to the recently published gene cluster for BE-54017, a closely related indenotryptoline natural product. The cladoniamide gene cluster differs from the BE-54017 gene cluster in gene organization and in the absence of one N-methyltransferase gene but otherwise contains close homologs to all genes in the BE-54017 cluster. Both gene clusters encode enzymes needed for the construction of an indolocarbazole core, as well as flavin-dependent enzymes putatively involved in generating the indenotryptoline scaffold from an indolocarbazole. These two bis-indolic gene clusters exemplify the diversity of biosynthetic routes that begin from the oxidative dimerization of two molecules of L-tryptophan, highlight enzymes for further study, and provide new opportunities for combinatorial engineering.     

Biosynthesis of 2'-O-methylmyxalamide D in the myxobacterium Cystobacter fuscus: a polyketide synthase-nonribosomal peptide synthetase system for the myxalamide D skeleton and a methyltransferase for the final O-methylation

The biosynthetic gene cluster for the polyene antifungal antibiotic, 2'-O-methylmyxalamide D, was cloned from myxobacterium Cystobacter fuscus AJ-13278. A sequence analysis of the 12.8-kb region in the gene cluster revealed the presence of two type I polyketide synthase genes, mmxB and mmxC. The involvement of these two genes in the biosynthesis of 2'-O-methylmyxalamide D was confirmed by a gene disruption experiments. In addition, an S-adenosylmethionine-dependent methyltransferase gene (mmxM) was found downstream of the gene cluster and demonstrated, by a gene disruption analysis, to be responsible for converting the known unmethylated precursor, myxalamide D, into 2'-O-methylmyxalamide D.     

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.     

From genetic diversity to metabolic unity: studies on the biosynthesis of aurafurones and aurafuron-like structures in myxobacteria and streptomycetes

The myxobacterial polyketide secondary metabolites aurafuron A and B were identified by genome mining in the myxobacterial strain Stigmatella aurantiaca DW4/3-1. The compounds contain an unusual furanone moiety and resemble metabolites isolated from soil-dwelling and marine actinobacteria, a fungus and mollusks. We describe here the cloning and functional analysis of the aurafuron biosynthetic gene cluster, including site-directed mutagenesis and feeding studies using labeled precursors. The polyketide core of the aurafurones is assembled by a modular polyketide synthase (PKS). As with many such systems described from myxobacteria, the aurafuron PKS exhibits a number of unusual features, including the apparent iterative use of a module, redundant modules and domains, a trans acting dehydratase and the absence of a terminal thioesterase domain. Four oxidoreductases are encoded within the gene locus, some of which likely participate in formation of the furanone moiety via a Baeyer-Villiger type oxidation. Indeed, inactivation of a gene encoding a cytochrome P₄₅₀ monooxygenase completely abolished production of both compounds. We also compare the complete gene locus to biosynthetic gene clusters from two Streptomyces sp., which produce close structural analogues of the aurafurones. A portion of the post-PKS biosynthetic machinery is strikingly similar in all three cases, in contrast to the PKS genes, which are highly divergent. Phylogenetic analysis of the ketosynthase domains further indicates that the PKSs have developed independently (polyphyletically) during evolution. These findings point to a currently unknown but important biological function of aurafuron-like compounds for the producing organisms.     

Studies on the biosynthesis of the lipodepsipeptide antibiotic Ramoplanin A2

Ramoplanin, a non-ribosomally synthesized peptide antibiotic, is highly effective against several drug-resistant Gram-positive bacteria, including vancomycin-resistant Enterococcus faecium (VRE) and methicillin-resistant Staphylococcus aureus (MRSA), two important opportunistic human pathogens. Recently, the biosynthetic cluster from the ramoplanin producer Actinoplanes ATCC 33076 was sequenced, revealing an unusual architecture of fatty acid and non-ribosomal peptide synthetase biosynthetic genes (NRPSs). The first steps towards understanding how these biosynthetic enzymes cooperatively interact to produce the depsipeptide product are expression and isolation of each enzyme to probe its specificity and function. Here we describe the successful production of soluble enzymes from within the ramoplanin locus and the confirmation of their specific role in biosynthesis. These methods may be broadly applicable to the production of biosynthetic enzymes from other natural product biosynthetic gene clusters, especially those that have been refractory to production in heterologous hosts despite standard expression optimization methods.     

Genetics of lipopolysaccharide biosynthesis in enteric bacteria.

From a historical perspective, the study of both the biochemistry and the genetics of lipopolysaccharide (LPS) synthesis began with the enteric bacteria. These organisms have again come to the forefront as the blocks of genes involved in LPS synthesis have been sequenced and analyzed. A number of new and unanticipated genes were found in these clusters, indicating a complexity of the biochemical pathways which was not predicted from the older studies. One of the most dramatic areas of LPS research has been the elucidation of the lipid A biosynthetic pathway. Four of the genes in this pathway have now been identified and sequenced, and three of them are located in a complex operon which also contains genes involved in DNA and phospholipid synthesis. The rfa gene cluster, which contains many of the genes for LPS core synthesis, includes at least 17 genes. One of the remarkable findings in this cluster is a group of several genes which appear to be involved in the synthesis of alternate rough core species which are modified so that they cannot be acceptors for O-specific polysaccharides. The rfb gene clusters which encode O-antigen synthesis have been sequenced from a number of serotypes and exhibit the genetic polymorphism anticipated on the basis of the chemical complexity of the O antigens. These clusters appear to have originated by the exchange of blocks of genes among ancestral organisms. Among the large number of LPS genes which have now been sequenced from these rfa and rfb clusters, there are none which encode proteins that appear to be secreted across the cytoplasmic membrane and surprisingly few which encode integral membrane proteins or proteins with extensive hydrophobic domains. These data, together with sequence comparison and complementation experiments across strain and species lines, suggest that the LPS biosynthetic enzymes may be organized into clusters on the inner surface of the cytoplasmic membrane which are organized around a few key membrane proteins.     

Genetic manipulation of the biosynthetic process leading to phoslactomycins, potent protein phosphatase 2A inhibitors

Phoslactomycins (PLMs) represent an unusual structural class of natural products secreted by various streptomycetes, containing an α,β-unsaturated δ-lactone, an amino group, phosphate ester, conjugated diene and a cyclohexane ring. Phosphazomycins, phospholines and leustroducsins contain the same structural moieties, varying only in the acyl substituent at the C-18 hydroxyl position. These compounds possess either antifungal or antitumor activities or both. The antitumor activity of the PLM class of compounds has been attributed to a potent and selective inhibition of protein phosphatase 2A (PP2A). The cysteine-269 residue of PP2Ac-subunit has been shown to be the site of covalent modification by PLMs. In this article, we review previous work on the isolation, structure elucidation and biological activities of PLMs and related compounds and current status of our work on both PLM stability and genetic manipulation of the biosynthetic process. Our work has shown that PLM B is surprisingly stable in solution, with a pH optimum of 6. Preliminary biosynthetic studies utilizing isotopically labeled shikimic acid and cyclohexanecarboxylic acid (CHC) suggested PLM B to be a polyketide-type antibiotic synthesized using CHC as a starter unit. Using a gene (chcA) from a set of CHC-CoA biosynthesis genes from Streptomyces collinus as a probe, a 75 kb region of 29 ORFs encoding PLM biosynthesis was located in the genome of Streptomyces sp. strain HK803. Analysis and subsequent manipulation of plmS ₂ and plmR ₂ in the gene cluster has allowed for rational engineering of a strain that produces only one PLM analog, PLM B, at ninefold higher titers than the wild type strain. A strain producing PLM G (the penultimate intermediate in PLMs biosynthesis) has also been generated. Current work is aimed at selective in vitro acylation of PLM G with various carboxylic acids and a precursor-directed biosynthesis in a chcA deletion mutant with the aim of generating novel PLM analogs.