Study of the bldG locus suggests that an anti-anti-sigma factor and an anti-sigma factor may be involved in Streptomyces coelicolor antibiotic production and sporulation

A cloned 2.5 kb DNA fragment that can restore antibiotic production and sporulation to a bldG mutant encodes a 113 aa protein showing similarity to a family of anti-anti-sigma factors from Bacillus and Staphylococcus; and the deduced product of a closely spaced downstream ORF, designated ORF3, shows similarity to cognate anti-sigma factors. The homologues in Bacillus regulate the activity of sporulation- and stress-response-specific sigma factors. However, there is no sigma factor gene near bldG and ORF3. bldG is transcribed both as a monocistronic and a polycistronic mRNA, the latter including the downstream ORF3 gene. The two transcripts were present at all time points during growth and both were upregulated when aerial mycelium and pigmented antibiotics were seen. At all time points, the monocistronic bldG transcript was two- to threefold more abundant than the polycistronic transcript. Mapping of the mRNA 5' ends indicated that bldG transcription is initiated from two transcription start sites located 82 and 123 bp upstream of the bldG translation start. A constructed bldG null mutant had the same phenotype as previously isolated bldG point mutations, some of which were shown to have potentially significant base changes within bldG. When compared to the wild-type strain, the null mutant showed no differences in the levels of transcription from the two bldG promoters. These results suggest that bldG is not involved in autoregulation.