The Streptomyces coelicolor sporulation-specific sigma WhiG form of RNA polymerase transcribes a gene encoding a ProX-like protein that is dispensable for sporulation.

In the non-motile mycelial organism Streptomyces coelicolor A3(2), the sporulation gene whiG encodes a protein that closely resembles RNA polymerase sigma factors such as sigma D of Bacillus subtilis, which mainly control motility and chemotaxis genes. Here, we show that the whiG gene product, purified from an Escherichia coli strain carrying an expression construct, could activate E. coli core RNA polymerase in vitro to transcribe a sigma D-dependent motility-related promoter from B. subtilis. Such RNA polymerase holoenzyme preparations could also transcribe from an S. coelicolor promoter, PTH4, previously shown to require an intact whiG gene for in-vivo transcription. The in-vivo dependence on whiG was therefore shown to be direct. Unusually, the initiation of PTH4 transcription in vitro depended on the provision of appropriate dinucleotides. The whiG-dependent PTH4 transcription unit consisted of a single gene, orfTH4. Sequence comparisons suggested that the gene product was a member of a small group of proteins that include the B. subtilis and E. coli ProX proteins. Though none of these proteins shared more than about 30% of extended primary sequence identity, they had similar size and hydropathy profiles, and could be aligned end to end to reveal a mosaic of similarities. The ProX proteins of B. subtilis and E. coli are implicated in glycine betaine transport in response to hyperosmotic stress. However, disruption of orfTH4 did not cause any obvious phenotypic changes in growth or development on media of varying osmotic strengths.