Feed-Forward Regulation of Microbisporicin Biosynthesis in Microbispora corallina

Lantibiotics are ribosomally synthesized, posttranslationally modified peptide antibiotics. Microbisporicin is a potent lantibiotic produced by the actinomycete Microbispora corallina and contains unique chlorinated tryptophan and dihydroxyproline residues. The biosynthetic gene cluster for microbisporicin encodes several putative regulatory proteins, including, uniquely, an extracytoplasmic function (ECF) sigma factor, sigma(MibX), a likely cognate anti-sigma factor, MibW, and a potential helix-turn-helix DNA binding protein, MibR. Here we examine the roles of these proteins in regulating microbisporicin biosynthesis. S1 nuclease protection assays were used to determine transcriptional start sites in the microbisporicin gene cluster and confirmed the presence of the likely ECF sigma factor -10 and -35 sequences in five out of six promoters. In contrast, the promoter of mibA, encoding the microbisporicin prepropeptide, has a typical Streptomyces vegetative sigma factor consensus sequence. The ECF sigma factor sigma(MibX) was shown to interact with the putative anti-sigma factor MibW in Escherichia coli using bacterial two-hybrid analysis. sigma(MibX) autoregulates its own expression but does not directly regulate expression of mibA. On the basis of quantitative reverse transcriptase PCR (qRT-PCR) data, we propose a model for the biosynthesis of microbisporicin in which MibR functions as an essential master regulator and the ECF sigma factor/anti-sigma factor pair, sigma(MibX)/MibW, induces feed-forward biosynthesis of microbisporicin and producer immunity.