Bacillus subtilis sigma 28 and Escherichia coli sigma 32 (htpR) are minor sigma factors that display an overlapping promoter specificity.

Bacillus subtilis sigma 28-specific promoters (P28) are utilized by a minor form of B. subtilis RNA polymerase (sigma 28RNA polymerase) and not by the predominant RNA polymerases of B. subtilis (sigma 43 RNA polymerase) or Escherichia coli (sigma 70 RNA polymerase). However B. subtilis P28 are effective promoters in E. coli. This transcription depends on the E. coli htpR+ gene. Similarly, the E. coli rpoD heat shock promoter which is under control of htpR is used in vitro by B. subtilis sigma 28 RNA polymerase. These observations are explained by the fact that E. coli htpR is a minor sigma factor (sigma 32) which shares an overlapping promoter specificity with B. subtilis sigma 28 RNA polymerase. Hence control of bacterial regulons by minor sigma factors is not restricted to Bacilli, or bacteria that carry out a complex differentiation process, but is probably a general, regulatory mechanism in prokaryotes. Transcription from B. subtilis P28 in E. coli does not depend on a heat shock. This suggests that the sequences that control E. coli sigma 32 action are separate from those that control the heat shock regulon. Hence the action of polymerases controlled by minor sigma factors in both B. subtilis and E. coli appears to be controlled by a separate set of regulatory factors.