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

Abstract

We have identified a second RpoH homolog, RpoHII, in the α-proteobacterium Rhodobacter sphaeroides. Primary amino acid sequence comparisons demonstrate that R. sphaeroides RpoHII belongs to a phylogenetically distinct group with RpoH orthologs from α-proteobacteria that contain two rpoH genes. Like its previously identified paralog, RpoH I, RpoHII is able to complement the temperature-sensitive phenotype of an Escherichia coli σ32 (rpoH) mutant. In addition, we show that recombinant RpoHI and RpoHII each transcribe two E. coli σ32-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, groESL1, rpoD P HS, dnaK P1, hslO, and ecfE), all of which resemble the E. coli σ32 promoter consensus. While RpoHI reconstituted with R. sphaeroides core RNA polymerase transcribed all six promoters, RpoH II produced detectable transcripts from only four promoters (cycA P1, groESL1, hslO, and ecfE). These results, in combination with previous work demonstrating that an RpoHI mutant mounts a typical heat shock response, suggest that while RpoHI and RpoHII have redundant roles in response to heat, they may also have roles in response to other environmental stresses. Copyright © 2006, American Society for Microbiology. All Rights Reserved.