A new RNA polymerase sigma factor, σ^F is required for the late stages of morphological differentiation in Streptomyces spp.

A gene (sigF) encoding a new sigma factor was isolated from Streptomyces aureofaciens using a degenerate oligonucleotide probe designed from the GLI(KDNE)A motif lying within the well‐conserved region 2.2 of the eubacterial σ⁷⁰ family. Homologues were present in other Streptomyces spp., and that of the genetically well‐studied Streptomyces coelicolor A3(2) was also cloned. The nucleotide sequences of the two sigF genes were determined and shown to encode primary translation products of 287 (S. coelicolor) and 295 (S. aureofaciens) amino acid residues, both showing greatest similarity to σ^B of Bacillus subtilis. However, while σ^B is involved in stationary‐phase gene expression and in the general stress response in B. subtilis, σ^F affects morphological differentiation in Streptomyces, Disruption of sigF did not affect vegetative growth but did cause a whi mutant phenotype. Microscopic examination showed that the sigF mutant produced spores that were smaller and deformed compared with those of the wild type, that the spore walls were thinner and sensitive to detergents and that in sigF mutant spores the chromosome failed to condense. σ^F is proposed to control the late stages of spore development in Streptomyces.