Cloning, sequencing, and functional analysis of an iterative type I polyketide synthase gene cluster for biosynthesis of the antitumor chlorinated polyenone neocarzilin in "Streptomyces carzinostaticus".

Neocarzilins (NCZs) are antitumor chlorinated polyenones produced by "Streptomyces carzinostaticus" var. F-41. The gene cluster responsible for the biosynthesis of NCZs was cloned and characterized. DNA sequence analysis of a 33-kb region revealed a cluster of 14 open reading frames (ORFs), three of which (ORF4, ORF5, and ORF6) encode type I polyketide synthase (PKS), which consists of four modules. Unusual features of the modular organization is the lack of an obvious acyltransferase domain on modules 2 and 4 and the presence of longer interdomain regions more than 200 amino acids in length on each module. Involvement of the PKS genes in NCZ biosynthesis was demonstrated by heterologous expression of the cluster in Streptomyces coelicolor CH999, which produced the apparent NCZ biosynthetic intermediates dechloroneocarzillin A and dechloroneocarzilin B. Disruption of ORF5 resulted in a failure of NCZ production, providing further evidence that the cluster is essential for NCZ biosynthesis. Mechanistic consideration of NCZ formation indicates the iterative use of at least one module of the PKS, which subsequently releases its product by decarboxylation to generate an NCZ skeleton, possibly catalyzed by a type II thioesterase encoded by ORF7. This is a novel type I PKS system of bacterial origin for the biosynthesis of a reduced polyketide chain. Additionally, the protein encoded by ORF3, located upstream of the PKS genes, closely resembles the FADH(2)-dependent halogenases involved in the formation of halometabolites. The ORF3 protein could be responsible for the halogenation of NCZs, presenting a unique example of a halogenase involved in the biosynthesis of an aliphatic halometabolite.