The transcriptional regulator gene phrR in Sinorhizobium meliloti WSM419 is regulated by low pH and other stresses

The phrR gene in Sinorhizobium meliloti (previously known as Rhizobium meli loti) WSM419, directly downstream from actA, is induced by low ph or certai n stresses (e.g. high concentrations of Zn2+, Cu2+, H2O2 or ethanol), but n ot in stationary phase or by other stresses (e.g. phosphate limitation, ele vated temperature, high concentrations of sucrose or iron). A DNA fragment containing the wild-type phrR gene could not be cloned and inverse PCR was therefore used to amplify a 3.5 kb BamHI fragment containing phrR from the mutant S. meliloti TC2-6 (actA::Tn5). DMA fragments from a BamHI/SalI diges t of the amplified product were cloned into pUK21 and sequenced. The phrR o pen reading frame contiguous to actA appears to code for a 15.2 kDa protein showing significant identity with the proteins encoded by y4wC and y4aM in Rhizobium sp. NGR234. All three proteins resemble transcriptional regulato rs in containing a DNA-binding helix-turn-helix motif similar to that repor ted for URF4 in Rhodospirillum rubrum and repressors in coliphage.