Role of σH paralogs in intracellular melanin formation and spore development in Streptomyces griseus

Streptomyces griseus possesses multiple stress-response sigma factors including σH. Previously, we have suggested that σH and related sigma factors are involved in the developmental control of S. griseus. Herein, we studied the role of two σH paralogs—σF and σN—which are encoded in tandem coding sequences of sigF–sigN in S. griseus [σN has been described as σL previously (Gene 320:127, 2003)]. A sigF mutant produced decreased levels of intracellular melanin and formed irregular spores. A triple mutant for sigHNF exhibited defective melanin production. While sigN was transcribed by three tandem promoters during the early to late growth phases, sigF was transcribed in the late developmental phase by a single promoter. The activity of the promoter preceding the rpp operon (Prpp), which is responsible for the intracellular melanin biosynthesis, was decreased in the sigF mutant and abolished in the sigHNF, adpA and A-factor biosynthesis mutants. The in vitro transcription assay demonstrated that EσF transcribed the rpp promoter. Both EσF and EσN transcribed a σH-dependent promoter that preceded the sigH operon, and their activities were repressed by the addition of RshA, an anti-σH protein. Overall, the results suggest that the three sigma factors have similar functions and that they are required for spore development and pigmentation. The transcription of the rpp operon is regulated both by the stress-response sigma factors and the A-factor regulatory cascade.