Identification of the heat-shock sigma factor RpoH and a second RpoH-like protein in Sinorhizobium meliloti

Hybridization to a PCR product derived from conserved sigma-factor sequence s led to the identification of two Sinorhizobium meliloti DNA segments that display significant sequence similarity to the family of rpoH genes encodi ng the sigma (32) (RpoH) heat-shock transcription factors. The first gene, rpoH1, complements an Escherichia coli rpoH mutation. Cells containing an r poH1 mutation are impaired in growth at 37 degreesC under free-living condi tions and are defective in nitrogen fixation during symbiosis with alfalfa. A plasmid-borne rpoH1-gusA fusion increases in expression upon entry of th e culture into the stationary phase of growth. The second gene, designated rpoH2, is 42% identical to the S. meliloti rpoH1 gene. Cells containing an rpoH2 mutation have no apparent phenotype under free-living conditions or d uring symbiosis with the host plant alfalfa. An rpoH2-gusA fusion increases in expression during the stationary phase of growth. The presence of two r poH-like sequences in S. meliloti is reminiscent of the situation in Bradyr hizobium japonicum, which has three rpoH genes.