THE POSITIONS OF THE SIGMA-FACTOR GENES, WHIG AND SIGF, IN THE HIERARCHY CONTROLLING THE DEVELOPMENT OF SPORE CHAINS IN THE AERIAL HYPHAE OF STREPTOMYCES-COELICOLOR A3(2)

whiG and sigF encode RNA polymerase sigma factors required for sporula tion in the aerial hyphae of Streptomyces coelicolor. Their expression was analysed during colony development in wild-type and sporulation-d efective whi mutant strains. Each gene was transcribed from a single p romoter. Unexpectedly, whiG mRNA was present at all time points, inclu ding those taken prior to aerial mycelium formation; this suggests tha t whiG may be regulated post-transcriptionally. Transcription of whiG did not depend upon any of the six known 'early' whi genes required fo r sporulation septum formation (whiA, B, G, H, I and J), placing it at the top of the hierarchy of whi loci, sigF expression appeared to be regulated at the level of transcription; sigF transcripts were detecte d transiently when sporulation septa were observed in the aerial hypha e. Transcription of sigF depended upon all six of the early whi genes, including whiG. The sigF promoter does not resemble the consensus seq uence established for sigma(WhiG)-dependent promoters and E sigma(WhiG ) did not transcribe from the sigF promoter in vitro, Consequently, th e genetic dependence of sigF upon whiG is very likely to be indirect. These results show that there is a hierarchical relationship between s igma factors required for Streptomyces sporulation and also that at le ast five other genes are involved in this transcriptional network.