Regulation of σ^B by an anti- and an anti-anti-sigma factor in Streptomyces coelicolor in response to osmotic stress

σ^B, a homolog of stress-responsive σ^B of Bacillus subtilis, controls both osmoprotection and differentiation in Streptomyces coelicolor A3 (2). Its gene is preceded by rsbA and rsbB genes encoding homologs of an anti-sigma factor, RsbW, and its antagonist, RsbV, of B. subtilis, respectively. Purified RsbA bound to σ^B and prevented σ^B-directed transcription from the sigBp1 promoter in vitro. An rsbA-null mutant exhibited contrasting behavior to the sigB mutant, with elevated sigBp1 transcription, no actinorhodin production, and precocious aerial mycelial formation, reflecting enhanced activity of σ^B in vivo. Despite sequence similarity to RsbV, RsbB lacks the conserved phosphorylatable serine residue and its gene disruption produced no distinct phenotype. RsbV (SCO7325) from a putative six-gene operon (rsbV-rsbR-rsbS-rsbT- rsbU1-rsbU) was strongly induced by osmotic stress in a σ^B- dependent manner. It antagonized the inhibitory action of RsbA on σ^B-directed transcription and was phosphorylated by RsbA in vitro. These results support the hypothesis that the rapid induction of σ^B target genes by osmotic stress results from modulation of σ^B activity by the kinase-anti-sigma factor RsbA and its phosphorylatable antagonist RsbV, which function by a partner-switching mechanism. Amplified induction could result from a rapid increase in the synthesis of both σ^B and its inhibitor antagonist.