Activity of Rhodobacter sphaeroides RpoH II , a Second Member of the Heat Shock Sigma Factor Family

Abstract

ABSTRACT We have identified a second RpoH homolog, RpoH II , in the α-proteobacterium Rhodobacter sphaeroides . Primary amino acid sequence comparisons demonstrate that R. sphaeroides RpoH II belongs to a phylogenetically distinct group with RpoH orthologs from α-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 σ 32 ( rpoH ) mutant. In addition, we show that recombinant RpoH I and RpoH II each transcribe two E. coli σ 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, hslO , and ecfE ), all of which resemble the E. coli σ 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 , hslO , 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.