Two functional domains conserved in major and alternate bacterial sigma factors.

Sequences of the sigma factors of Escherichia coli and Bacillus subtilis were aligned with the sequences of two sigma-like proteins, HtpR, involved in the expression of heat-shock genes in E. coli, and SpoIIG, necessary for endospore formation in B. subtilis. An internal region is highly conserved in the four proteins and is proposed to be involved in binding of sigma factors to core RNA polymerase. The carboxy-terminal part of the four proteins presents the characteristic structure found in several prokaryotic DNA-binding proteins and is proposed to be involved in promoter recognition.