Co-factor independent oxidases ncnN and actVA-3 are involved in the dimerization of benzoisochromanequinone antibiotics in naphthocyclinone and actinorhodin biosynthesis

Streptomyces produce complex bioactive secondary metabolites with remarkable chemical diversity. Benzoisochromanequinone polyketides actinorhodin and naphthocyclinone are formed through dimerization of half-molecules via single or double carbon-carbon bonds, respectively. Here we sequenced the genome of S. arenae DSM40737 to identify the naphthocyclinone gene cluster and established heterologous production in S. albus J1074 by utilizing direct cluster capture techniques. Comparative sequence analysis uncovered ncnN and ncnM gene products as putative enzymes responsible for dimerization. Inactivation of ncnN that is homologous to atypical co-factor independent oxidases resulted in the accumulation of fogacin, which is likely a reduced shunt product of the true substrate for naphthocyclinone dimerization. In agreement, inactivation of the homologous actVA-3 in S. coelicolor M145 also led to significantly reduced production of actinorhodin. Previous work has identified the NAD(P)H-dependent reductase ActVA-4 as the key enzyme in actinorhodin dimerization, but surprisingly inactivation of the homologous ncnM did not abolish naphthocyclinone formation and the mutation may have been complemented by an endogenous gene product. Our data suggests that dimerization of benzoisochromanequinone polyketides require two-component reductase-oxidase systems.

Structure, activity, synthesis and biosynthesis of aryl-C-glycosides

The focus of this review is to highlight the structure, bioactivity and biosynthesis of naturally occurring aryl-C-glycosides. General synthetic methods and their relevance to proposed biochemical mechanisms for the aryl-C-glycoside bond formation are also presented.

Isolation and characterization of the naphthocyclinone gene cluster from Streptomyces arenae DSM 40737 and heterologous expression of the polyketide synthase genes

Streptomyces arenae produces the aromatic polyketide naphthocyclinone, which exhibits activity against Gram-positive bacteria. A cosmid clone containing the putative naphthocyclinone gene cluster was isolated from a genomic library of S. arenae by hybridization with a conserved region from the actinorhodin PKS of S. coelicolor. Sequence analysis of a 5.5-kb DNA fragment, which hybridizes with the actI probe, revealed three open reading frames coding for the minimal polyketide synthase. A strong sequence similarity was found to several previously described ketosynthases, chain length factors and acyl carrier proteins from other polyketide gene clusters. An additional open reading frame downstream of the PKS genes of S. arenae showed 53% identity to act VII probably encoding an aromatase. Another open reading frame was identified in a region of 1.436 bp upstream of the PKS genes, which, however, had no similarity to known genes in the database. Approximately 8 kb upstream of the PKS genes, a DNA fragment was identified that hybridizes to an actVII--actIV specific probe coding for a cyclase and a putative regulatory protein, respectively. Disruption of the proposed naphthocyclinone gene cluster by insertion of a thiostrepton resistance gene completely abolished production of naphthocyclinones in the mutant strain, showing that indeed the naphthocyclinone gene cluster had been isolated. Heterologous expression of the minimal PKS genes in S. coelicolor CH999 in the presence of the act ketoreductase led to the production of mutactin and dehydromutactin, indicating that the S. arenae polyketide synthase forms a C-16 backbone that is subsequently dimerized to build naphthocyclinone. The functions of the proposed cyclase and aromatase were examined by coexpression with genes from different polyketide core producers.