The stress-response sigma factor σ^H controls the expression of ssgB, a homologue of the sporulation-specific cell division gene ssgA, in Streptomyces coelicolor A3(2)

Abstract.

By using a previously established method for the identification of promoters recognized by a particular sigma factor of RNA polymerase, we identified a promoter in Streptomyces coelicolor A3(2) that is recognized by a heterologous RNA polymerase containing the late sporulation-specific sigma factor σ^F. The promoter directed the expression of a gene named ssgB, which is related to the sporulation-specific cell division gene ssgA. These genes, together with three others, constitute a new family of paralogous genes specific for Streptomyces. S1-nuclease mapping using RNA prepared from an Escherichia coli strain that expresses ssgB under the control of σ^F, and from S. coelicolor A3(2) at various developmental stages, identified identical transcription start points in both strains, corresponding to the promoter ssgBp. The promoter is developmentally regulated in S. coelicolor: it is induced at the time of aerial mycelium formation and is most active during sporulation. However, the level of the ssgB transcript was unaffected in a sigF mutant of S. coelicolor A3(2). Interestingly, the level of the transcript was substantially reduced in an S. coelicolor strain that was mutant for the sigH gene, which encodes a stress-response sigma factor (σ^H) that is essential for sporulation in S. coelicolor A3(2). This dependence of ssgB upon sigH was confirmed by an in vitro transcription assay, in which σ^H, in the presence of the S. coelicolor core RNA polymerase, was able to recognize ssgBp. These results suggest that the S. coelicolor ssgB gene is under the control of the stress-response σ^H. Transcription of ssgB was investigated in S. coelicolor A3(2) mutants with lesions in each of six known early whi genes required for sporulation septation. Expression of ssgB was deregulated in three of the mutants (whiA, whiI, and whiJ). Based on these data, it is proposed that the ssgB gene product plays a role in the developmental process in S. coelicolor A3(2).