A NEW RNA-POLYMERASE SIGMA-FACTOR, SIGMA(F), IS REQUIRED FOR THE LATESTAGES OF MORPHOLOGICAL-DIFFERENTIATION IN STREPTOMYCES SPP

A gene (sigF) encoding a new sigma factor was isolated from Streptomyc es aureofaciens using a degenerate oligonucleotide probe designed from the GLI(KDNE)A motif lying within the well-conserved region 2.2 of th e eubacterial sigma(70) family. Homologues were present in other Strep tomyces spp., and that of the genetically well-studied Streptomyces co elicolor A3(2) was also cloned. The nucleotide sequences of the two si gF genes were determined and shown to encode primary translation produ cts of 287 (S. coelicolor) and 295 (S. aureofaciens) amino acid residu es, both showing greatest similarity to sigma(B) of Bacillus subtilis. However, while sigma(B) is involved in stationary-phase gene expressi on and in the general stress response in B. subtilis, sigma(F) affects morphological differentiation in Streptomyces. Disruption of sigF did not affect vegetative growth but did cause a whi mutant phenotype. Mi croscopic 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 s igF mutant spores the chromosome failed to condense. sigma(F) is propo sed to control the late stages of spore development in Streptomyces.