Genetic analysis of the biosynthesis of the pyrrole and carbamoyl moieties of coumermycin A1 and novobiocin

The aminocoumarin antibiotic coumermycin AI contains a central and two terminal pyrrole moieties. The coumermycin gene cluster in Streptomyces rishiriensis contains three genes (couN3, couN4 and couN5) that show sequence similarity to genes involved in the biosynthesis of the pyrrole moieties of pyoluteorin in Pseudomonas fluorescens and of undecylprodiginine in S. coelicolor. The gene couN3, which codes for a putative L-prolyl-S-PCP dehydrogenase, and the gene couN4, which encodes a putative L-prolyl-AMP ligase, were disrupted using in-frame deletion and insertional inactivation, respectively. HPLC analysis of culture extracts showed that formation of the two terminal pyrrole moieties was abolished in the couN3(-) and couN4(-) mutants. The mutants accumulated coumermycin D, which contains only the central pyrrole moiety. This result not only confirmed the involvement of couN3 and couN4 in the biosynthesis of the terminal pyrrole-2-carboxylic acid moieties of coumermycin A,, but also indicated, for the first time, that the central 3-methylpyrrole-2,4 -dicarboxylic acid unit of the coumermycins is formed by a biosynthetic pathway that differs from that used to assemble the terminal pyrrole moieties. novN, a putative carbamoyl transferase gene from the gene cluster for novobiocin biosynthesis in S. spheroides was expressed in the couN3- mutant. This led to the formation of biscarbamoylated coumermycin D, a novel compound of the coumermycin series.