The Bradyrhizobium japonicum rpoH(1) gene encoding a sigma(32)-like protein is part of a unique heat shock gene cluster together with groESL(1) and three small heat shock genes

The heat shock response of Bradyrhizobium japonicum is controlled by a complex network involving two known regulatory systems. While some heat shock genes are controlled by a highly conserved inverted-repeat structure (CIRCE), others depend on a sigma(32)-type heat shock sigma factor. Using Western blot (immunoblot) analysis, we confirmed the presence of a sigma(32)-like protein in B. japonicum and defined its induction pattern after heat shock A B. japonicum rpoH-like gene (rpoH(1)) was cloned by complementation of an Escherichia coli strain lacking sigma(32), A knockout mutation in rpoH(1) did not abolish sigma(32) production in B. japonicum, and the rpoH(1) mutant showed the wild-type growth phenotype, suggesting the presence of multiple rpoH homologs in this bacterium. Further characterization of the rpoH(1) gene region revealed that the rpoH(1) gene is located in a heat shock gene cluster together with the previously characterized groESL(1) operon and three genes encoding small heat shock proteins in the following arrangement: groES(1), groEL(1), hspA, rpoH(1), hspB, and hspC, Three heat-inducible promoters are responsible for transcription of the six genes as three bicistronic operons. A sigma(32)-dependent promoter has previously been described upstream of the groESL(1) operon. Although the hspA-rpoH(1) and hspBC operons were clearly heat inducible, they were preceded by sigma(70)-like promoters. Interestingly, a stretch of about 100 bp between the transcription start site and the start codon of the first gene in each of these two operons was nearly identical, making it a candidate for a regulatory element potentially allowing heat shock induction of sigma(70)-dependent promoters.