Yh. Cho et al., SigB, an RNA polymerase sigma factor required for osmoprotection and proper differentiation of Streptomyces coelicolor, MOL MICROB, 42(1), 2001, pp. 205-214
A gene (sigB) encoding an alternative sigma factor sigma (B) in Streptomyce
s coelicolor A3(2) was isolated and characterized. It encodes a polypeptide
of 281 amino acids (31546 Da) and is highly homologous to Bacillus subtili
s sigma (B). The sigB coding region is preceded by four open reading frames
(ORFs): dpsA, orfA, rsbB and rsbA in sequential order. RNA analyses reveal
ed that rsbB, rsbA and sigB constitute an operon (sigB operon). Transcripts
were produced constitutively from a promoter (sigBp2) upstream of the rsbB
coding region, contributing to the basal level expression of sigmaB protei
n. An inducible promoter (sigBp1) resembling the catB promoter (catBp) was
located between the rsbA and sigB coding regions. Transcripts from sigBp1 d
ramatically increased as cells differentiated on solid media, at the statio
nary phase in liquid media or by osmotic stresses similar to the behaviour
of catBp transcripts. Both catBp and sigBp1 promoters were recognized speci
fically by sigma (B)-containing RNA polymerase in vitro. Disruption of the
sigB gene abolished not only the differentiation-associated expression but
also the osmotic induction of the catB gene, indicating that catBp is under
the control of UB. The sigB mutant exhibited a similar phenotype to the ca
tB mutant, being sensitive to hyperosmolarity, blocked in forming aerial my
celium and with skewed antibiotic production. Therefore, we conclude that s
igma (B) ensures the proper differentiation and osmoprotection of S. coelic
olor cells, primarily via regulation of the expression of catalase B.