Identification and transcriptional characterization of the gene encoding the stress-response σ factor σHinStreptomyces coelicolorA3(2)

We cloned a new gene, sigH, encoding an alternative sigma factor in Streptomyces coelicolor A3(2). The deduced protein of 354 amino acids with an M(r) of 39486 showed greatest similarity to the sporulation sigma factor (sigma(F)) of S. coelicolor, general stress-response sigma(B) of Bacillus subtilis, and stationary-phase stress-response sigma(F) of Mycobacterium tuberculosis. Sequence analysis of the upstream region revealed an ORF encoding a protein (UshX) similar to several anti-sigma factors, and short ORF (UshY) containing zinc-finger DNA binding motif. Transcriptional analysis revealed that all three genes are located on the same polycistronic transcript in order ushY, ushX, and sigH. Expression of the operon was directed by four promoters differentially expressed in the course of differentiation. The first (P1) constitutive promoter was located upstream of ushY. The other three promoters (P2, P3, and P4) were located upstream of ushX, and were differentially induced after various stress conditions. The magnitude of the induction was greatest after osmotic stress and heat shock.