Activity of Rhodobacter sphaeroides RpoHII, a second member of the heat shock sigma factor family

We have identified a second RpoH homolog, RpoH(II), in the alpha-proteobacterium Rhodobacter sphaeroides. Primary amino acid sequence comparisons demonstrate that R. sphaeroides RpoH(II) belongs to a phylogenetically distinct group with RpoH orthologs from alpha-proteobacteria that contain two rpoH genes. Like its previously identified paralog, RpoH(I), RpoH(II), is able to complement the temperature-sensitive phenotype of an Escherichia coli sigma(32) (rpoH) mutant. In addition, we show that recombinant RpoH(I) and RpoH(II) each transcribe two E. coli or sigma(32)-dependent promoters (rpoD P-HS and dnaK P1) when reconstituted with E. coli core RNA polymerase. We observed differences, however, in the ability of each sigma factor to recognize six R. sphaeroides promoters (cycA P1, groESL(1), rpoD P-HS, dnaK P1, hsl0, and ecfE), all of which resemble the E. coli or sigma(32) promoter consensus. While RpoH(I) reconstituted with R. sphaeroides core RNA polymerase transcribed all six promoters, RpoH(II) produced detectable transcripts from only four promoters (cycA P1, groESL(1), hsl0, and ecfE). These results, in combination with previous work demonstrating that an RpoH(I) mutant mounts a typical heat shock response, suggest that while RpoH(I), and RpoH(II), have redundant roles in response to heat, they may also have roles in response to other environmental stresses.