Cloning and characterization of a Myxococcus xanthus cytochrome P-450 hydroxylase required for biosynthesis of the polyketide antibiotic TA

Biosynthesis of polyketides by trans-AT polyketide synthases

This review discusses the biosynthesis of natural products that are generated bytrans-AT polyketide synthases, a family of catalytically versatile enzymes that represents one of the major group of proteins involved in the production of bioactive polyketides.

Global analysis of phase variation in Myxococcus xanthus

Myxococcus xanthus can vary its phenotype or 'phase' to produce colonies that contain predominantly yellow or tan cells that differ greatly in their abilities to swarm, survive and develop. Yellow variants are proficient at swarming (++) and tend to lyse in liquid during stationary phase. In contrast, tan variants are deficient in swarming (+) and persist beyond stationary phase. The phenotypes and transcriptomes of yellow and tan variants were compared with mutants affected in phase variation. Thirty-seven genes were upregulated specifically in yellow variants including those for production of the yellow pigment, DKxanthene. A mutant in DKxanthene synthesis produced non-pigmented (tan) colonies but still phase varied for swarming suggesting that pigmentation is not the cause of phase variation. Disruption of a gene encoding a HTH-Xre-like regulator, highly expressed in yellow variants, abolished pigment production and blocked the ability of cells to switch from a swarm ++ to a swarm (+) phenotype, showing that HTH-Xre regulates phase variation. Among the four genes whose expression was increased in tan variants was pkn14, which encodes a serine-threonine kinase that regulates programmed cell death in Myxococcus via the MrpC-MazF toxin-antitoxin complex. High levels of phosphorylated Pkn14 may explain why tan cells enjoy enhanced survival.

Biosynthesis of polyketides by trans-AT polyketide synthases

This review discusses the biosynthesis of natural products that are generated by trans-AT polyketide synthases, a family of catalytically versatile enzymes that have recently been recognized as one of the major group of proteins involved in the production of bioactive polyketides. 436 references are cited.

Cloning and biochemical characterization of the hectochlorin biosynthetic gene cluster from the marine cyanobacterium Lyngbya majuscula

Cyanobacteria, or blue-green algae, are a rich source of novel bioactive secondary metabolites that have potential applications as antimicrobial or anticancer agents or useful probes in cell biology studies. A Jamaican collection of the cyanobacterium Lyngbya majuscula has yielded several unique compounds including hectochlorin ( 1) and the jamaicamides A-C ( 5- 7). Hectochlorin has remarkable antifungal and cytotoxic properties. In this study, we have isolated the hectochlorin biosynthetic gene cluster ( hct) from L. majuscula to obtain details regarding its biosynthesis at the molecular genetic level. The genetic architecture and domain organization appear to be colinear with respect to its biosynthesis and consists of eight open reading frames (ORFs) spanning 38 kb. An unusual feature of the cluster is the presence of ketoreductase (KR) domains in two peptide synthetase modules, which are predicted to be involved in the formation of the two 2,3-dihydroxyisovaleric acid (DHIV) units. This biosynthetic motif has only recently been described in cereulide, valinomycin, and cryptophycin biosynthesis, and hence, this is only the second such report of an embedded ketoreductase in a cyanobacterial secondary metabolite gene cluster. Also present at the downstream end of the cluster are two cytochrome P450 monooxygenases, which are likely involved in the formation of the DHIV units. A putative halogenase, at the beginning of the gene cluster, is predicted to form 5,5-dichlorohexanoic acid.

Myxovirescin A Biosynthesis is Directed by Hybrid Polyketide Synthases/Nonribosomal Peptide Synthetase, 3-Hydroxy-3-Methylglutaryl-CoA Synthases, and trans-Acting Acyltransferases

Myxococcus xanthus DK1622 is shown to be a producer of myxovirescin (antibiotic TA) antibiotics. The myxovirescin biosynthetic gene cluster spans at least 21 open reading frames (ORFs) and covers a chromosomal region of approximately 83 kb. In silico analysis of myxovirescin ORFs in conjunction with genetic studies suggests the involvement of four type I polyketide synthases (PKSs; TaI, TaL, TaO, and TaP), one major hybrid PKS/NRPS (Ta-1), and a number of monofunctional enzymes similar to the ones involved in type II fatty-acid biosynthesis (FAB). Whereas deletion of either taI or taL causes a dramatic drop in myxovirescin production, deletion of both genes (DeltataIL) leads to the complete loss of myxovirescin production. These results suggest that both TaI and TaL PKSs might act in conjunction with a methyltransferase, reductases, and a monooxygenase to produce the 2-hydroxyvaleryl-S-ACP starter that is proposed to act as the biosynthetic primer in the initial condensation reaction with glycine. Polymerization of the remaining 11 acetates required for lactone formation is directed by 12 modules of Ta-1, TaO, and TaP megasynthetases. All modules, except for the first module of TaL, lack cognate acyltransferase (AT) domains. Furthermore, deletion of a discrete tandem AT-encoded by taV-blocks myxovirescin production; this suggests an "in trans" mode of action. To embellish the macrocycle with methyl and ethyl moieties, assembly of the myxovirescin scaffold is proposed to switch twice from PKS to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA)-like biochemistry during biosynthesis. Disruption of the S-adenosylmethionine (SAM)-dependent methyltransferase, TaQ, shifts production toward two novel myxovirescin analogues, designated myxovirescin Q(a) and myxovirescin Q(c). NMR analysis of purified myxovirescin Q(a) revealed the loss of the methoxy carbon atom. This novel analogue lacks bioactivity against E. coli.

An unusual beta-ketoacyl:acyl carrier protein synthase and acyltransferase motifs in TaK, a putative protein required for biosynthesis of the antibiotic TA in Myxococcus xanthus

The antibiotic TA of Myxococcus xanthus is produced by a type-I polyketide synthase mechanism. Previous studies have indicated that TA genes are clustered within a 36-kb region. The chemical structure of TA indicates the need for several post-modification steps, which are introduced to form the final bioactive molecule. These include three C-methylations, an O-methylation and a specific hydroxylation. In this study, we describe the genetic analysis of taK, encoding a specific polyketide beta-ketoacyl:acyl carrier protein synthase, which contains an unusual beta-ketoacyl synthase and acyltransferase motifs and is likely to be involved in antibiotic TA post-modification. Functional analysis of this beta-ketoacyl:acyl carrier protein synthase by specific gene disruption suggests that it is essential for the production of an active TA molecule.

Genetic and functional analysis of genes required for the post-modification of the polyketide antibiotic TA of Myxococcus xanthus

The antibiotic TA of Myxococcus xanthus is a complex macrocyclic polyketide, produced through successive condensations of acetate by a type I PKS (polyketide synthase) mechanism. The genes encoding TA biosynthesis are clustered on a 36 kb DNA fragment, which has been cloned and analysed. The chemical structure of TA and the mechanism by which it is synthesized indicate the need for several post-modification steps, which are introduced into the carbon chain of the polyketide to form the final bioactive molecule. These include the addition of several carbon atoms originating from acetate carbonyl, three C-methylations, O-methylation and a specific hydroxylation. This paper reports the analysis of five genes which are involved in the post-modification of TA. Their functional analysis, by specific gene disruption, suggests that they may be essential for the production of the active antibiotic. The characteristics and organization of the genes suggest that they may be involved in the addition of the carbon atoms which arise from acetate.

A nonessential signal peptidase II (Lsp) of Myxococcus xanthus might be involved in biosynthesis of the polyketide antibiotic TA

Myxococcus xanthus is a gram-negative soil bacterium that produces the polyketide antibiotic TA. In this study, we describe the analysis of an M. xanthus gene which encodes a homologue of the prolipoprotein signal peptidase II (SPase II; lsp). Overexpression of the M. xanthus SPase II in Escherichia coli confers high levels of globomycin resistance, confirming its function as an SPase II. The M. xanthus gene encoding the lsp homologue is nonessential for growth, as determined by specific gene disruption. It has been mapped to the antibiotic TA gene cluster, and the disrupted mutants do not produce the antibiotic, indicating a probable involvement in TA production. These results suggest the existence of more than one SPase II protein in M. xanthus, where one is a system-specific SPase II (for TA biosynthesis).