SummarySsgA plays an important role in the control of sporulation‐specific cell division and morphogenesis of streptomycetes, and ssgA null mutants have a rare conditionally non‐sporulating phenotype. In this paper we show that transcription of ssgA and of the upstream‐located ssgR, an iclR‐type regulatory gene, is developmentally regulated in Streptomyces coelicolor and activated towards the onset of sporulation. A constructed ssgR null mutant was phenotypically very similar to the ssgA mutant. The absence of ssgA transcription in this mutant is probably the sole cause of its sporulation deficiency, as wild‐type levels of sporulation could be restored by the SsgR‐independent expression of ssgA from the ermE promoter. Binding of a truncated version of SsgR to the ssgA promoter region showed that ssgA transcription is directly activated by SsgR; such a dependence of ssgA on SsgR in S. coelicolor is in clear contrast to the situation in S. griseus, where ssgA transcription is activated by A‐factor, and its control by the SsgR orthologue, SsfR, is far less important. Our failure to complement the ssgR mutant with S. griseus ssfR suggests functional differences between the genes. These observations may explain some of the major differences in developmental control between the phylogenetically divergent species S. coelicolor and S. griseus, highlighted in a recent microreview (Chater and Horinouchi (2003) Mol Microbiol 48: 9–15). Surprisingly, transcription of ssgA and ssgR is not dependent on the early whi genes (whiA, whiB, whiG, whiH, whiI and whiJ ).

Traag, B. A., Kelemen, G. H., & Van Wezel, G. P. (2004). Transcription of the sporulation gene ssgA is activated by the IclR‐type regulator SsgR in a whi‐independent manner in Streptomyces coelicolor A3(2). Molecular Microbiology, 53(3), 985–1000. Portico.