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Expression of thednaKJ andgroESL 1 heat shock operons ofBradyrhizobium japonicum depends on a ς32 -like transcription factor. Three such factors (RpoH1 , RpoH2 , and RpoH3 ) have previously been identified in this organism. We report here that they direct transcription from some but not all ς32 -type promoters when the respectiverpoH genes are expressed inEscherichia coli . All three RpoH factors were purified as soluble C-terminally histidine-tagged proteins, although the bulk of overproduced RpoH3 was insoluble. The purified proteins were recognized by an anti-E. coli ς32 serum. While RpoH1 and RpoH2 productively interacted withE. coli core RNA polymerase and producedE. coli groE transcript in vitro, RpoH3 was unable to do so.B. japonicum core RNA polymerase was prepared and reconstituted with the RpoH proteins. Again, RpoH1 and RpoH2 were active, and they initiated transcription at theB. japonicum groESL 1 anddnaKJ promoters. In all cases, the in vitro start site was shown to be identical to the start site determined in vivo. Promoter competition experiments revealed that theB. japonicum dnaKJ andgroESL 1 promoters were suboptimal for transcription by RpoH1 - or RpoH2 -containing RNA polymerase fromB. japonicum . In a mixture of different templates, theE. coli groESL promoter was preferred over any other promoter. Differences were observed in the specificities of both sigma factors towardB. japonicum rpoH -dependent promoters. We conclude that the primary function of RpoH2 is to supply the cell with DnaKJ under normal growth conditions whereas RpoH1 is responsible mainly for increasing the level of GroESL1 after a heat shock.

Promoter Selectivity of the Bradyrhizobium japonicum RpoH Transcription Factors In Vivo and In Vitro