The alternative sigma factor sigma(28) of the extreme thermophile Aquifex aeolicus restores motility to an Escherichia coli fliA mutant.

Sigma factor sigma(28) (sigma(F), FliA, SigD) directs RNA polymerase to transcribe the genes required for flagellar biosynthesis and chemotaxis in many bacteria, including Bacillus subtilis, Legionella pneumophila, Salmonella typhimurium, Escherichia coli, Yersinia enterolytica, Treponema maltophilum and Pseudomonas aeruginosa. Remarkably the fliA gene from the extreme thermophile Aquifex aeolicus restored motility to the E. coli mutant at relatively low temperature, albeit partially. This clearly demonstrates that A. aeolicus sigma(28) is able to direct RNA polymerase to E. coli sigma(28)-dependent promoters and take part in the complex interactions required to support transcription of the flagellar apparatus in vivo. The ability of A. aeolicus sigma(28) to function with mesophilic components shows that critical functional interactions made by these sigma factors are well conserved, and are not dependent upon high temperature. We over-produced and purified the sigma(28) protein and demonstrated binding to E. coli core RNA polymerase in vitro. In common with SigD from B. subtilis, but unlike most sigma factors, A. aeolicus sigma(28) showed DNA binding activity in vitro but there was no evidence of sequence specificity. We note that A. aeolicus sigma(28) is a good candidate for structural studies.