Metabolic Roles of a Rhodobacter sphaeroides Member of the ς(32) Family

We report the role of a gene (rpoH) from the facultative phototroph Rhodobacter sphaeroides that encodes a protein (ς(37)) similar to Escherichia coli ς(32) and other members of the heat shock family of eubacterial sigma factors. R. sphaeroides ς(37) controls genes that function during environmental stress, since an R. sphaeroides ΔRpoH mutant is ∼30-fold more sensitive to the toxic oxyanion tellurite than wild-type cells. However, the ΔRpoH mutant lacks several phenotypes characteristic of E. coli cells lacking ς(32). For example, an R. sphaeroides ΔRpoH mutant is not generally defective in phage morphogenesis, since it plates the lytic virus RS1, as well as its wild-type parent. In characterizing the response of R. sphaeroides to heat, we found that its growth temperature profile is different when cells generate energy by aerobic respiration, anaerobic respiration, or photosynthesis. However, growth of the ΔRpoH mutant is comparable to that of a wild-type strain under each of these conditions. The ΔRpoH mutant mounted a heat shock response when aerobically grown cells were shifted from 30 to 42°C, but it exhibited altered induction kinetics of ∼120-, 85-, 75-, and 65-kDa proteins. There was also reduced accumulation of several presumed heat shock transcripts (rpoD P(HS), groESL(1), etc.) when aerobically grown ΔRpoH cells were placed at 42°C. Under aerobic conditions, it appears that another sigma factor enables the ΔRpoH mutant to mount a heat shock response, since either RNA polymerase preparations from an ΔRpoH mutant, reconstituted Eς(37), or a holoenzyme containing a 38-kDa protein (ς(38)) each transcribed E. coli Eς(32)-dependent promoters. The lower growth temperature profile of photosynthetic cells is correlated with a difference in heat-inducible gene expression, since neither wild-type cells or the ΔRpoH mutant mount a typical heat shock response after such cultures were shifted from 30 to 37°C.