Identification of two regulatory genes involved in carbomycin biosynthesis in Streptomyces thermotolerans.

Abstract

Carbomycins are 16-membered macrolide antibiotics produced by Streptomyces thermotolerans ATCC 11416T. To characterize gene cluster responsible for carbomycin biosynthesis, the draft genome sequences for strain ATCC 11416T were obtained, from which the partial carbomycin biosynthetic gene cluster was identified. This gene cluster was approximately 40kb in length, and encoding 30 ORFs. Two putative transcriptional regulatory genes, acyB2 and cbmR, were inactivated by insertion of the apramycin resistance gene, and the resulting mutants were unable to produce carbomycin, thus confirming the involvement of two regulatory genes in carbomycin biosynthesis. Overexpression of acyB2 greatly improved the yield of carbomycin; however, overexpression of cbmR blocked carbomycin production. The qPCR analysis of the carbomycin biosynthetic genes in various mutants indicated that most genes were highly expressed in acyB2-overexpressing strains, but few expressed in cbmR-overexpressing strains. Furthermore, acyB2 co-expression with 4″-isovaleryltransferase gene (ist), resulted in efficient biotransformation of spiramycin into bitespiramycin in S. lividans TK24, whereas ist gene regulated by acyB2 and cbmR would cause the lower efficiency of spiramycin biotransformation. These results indicated that AcyB2 was a pathway-specific positive regulator of carbomycin biosynthesis. However, CbmR played a dual role in the carbomycin biosynthesis by acting as a positive regulator, and as a repressor at cbmR high expression levels.