Hybrid peptide-polyketide natural products: biosynthesis and prospects towards engineering novel molecules

Characterization of a gene cluster responsible for the biosynthesis of anticancer agent FK228 in Chromobacterium violaceum No. 968

A gene cluster responsible for the biosynthesis of anticancer agent FK228 has been identified, cloned, and partially characterized in Chromobacterium violaceum no. 968. First, a genome-scanning approach was applied to identify three distinctive C. violaceum no. 968 genomic DNA clones that code for portions of nonribosomal peptide synthetase and polyketide synthase. Next, a gene replacement system developed originally for Pseudomonas aeruginosa was adapted to inactivate the genomic DNA-associated candidate natural product biosynthetic genes in vivo with high efficiency. Inactivation of a nonribosomal peptide synthetase-encoding gene completely abolished FK228 production in mutant strains. Subsequently, the entire FK228 biosynthetic gene cluster was cloned and sequenced. This gene cluster is predicted to encompass a 36.4-kb DNA region that includes 14 genes. The products of nine biosynthetic genes are proposed to constitute an unusual hybrid nonribosomal peptide synthetase-polyketide synthase-nonribosomal peptide synthetase assembly line including accessory activities for the biosynthesis of FK228. In particular, a putative flavin adenine dinucleotide-dependent pyridine nucleotide-disulfide oxidoreductase is proposed to catalyze disulfide bond formation between two sulfhydryl groups of cysteine residues as the final step in FK228 biosynthesis. Acquisition of the FK228 biosynthetic gene cluster and acclimation of an efficient genetic system should enable genetic engineering of the FK228 biosynthetic pathway in C. violaceum no. 968 for the generation of structural analogs as anticancer drug candidates.

Structure and biosynthesis of heat-stable antifungal factor (HSAF), a broad-spectrum antimycotic with a novel mode of action

A screen for antifungal compounds from Lysobacter enzymogenes strain C3, a bacterial biological control agent of fungal diseases, has previously led to the isolation of heat-stable antifungal factor (HSAF). HSAF exhibits inhibitory activities against a wide range of fungal species and shows a novel mode of antifungal action by disrupting the biosynthesis of a distinct group of sphingolipids. We have now determined the chemical structure of HSAF, which is identical to that of dihydromaltophilin, an antifungal metabolite with a unique macrocyclic lactam system containing a tetramic acid moiety and a 5,5,6-tricyclic skeleton. We have also identified the genetic locus responsible for the biosynthesis of HSAF in strain C3. DNA sequencing of this locus revealed genes for a hybrid polyketide synthase-nonribosomal peptide synthetase (PKS-NRPS), a sterol desaturase, a ferredoxin reductase, and an arginase. The disruption of the PKS-NRPS gene generated C3 mutants that lost the ability to produce HSAF and to inhibit fungal growth, demonstrating a hybrid PKS-NRPS that catalyzed the biosynthesis of the unique macrolactam system that is found in many biologically active natural products isolated from marine organisms. In addition, we have generated mutants with disrupted sterol desaturase, ferredoxin reductase, and arginase and examined the metabolites produced in these mutants. The work represents the first study of the genetic basis for the biosynthesis of the tetramic acid-containing macrolactams. The elucidation of the chemical structure of HSAF and the identification of the genetic locus for its biosynthesis establish the foundation for future exploitation of this group of compounds as new fungicides or antifungal drugs.