The Bradyrhizobium japonicum rpoH1 gene encoding a sigma 32-like protein is part of a unique heat shock gene cluster together with groESL1 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 (rpoH1) was cloned by complementation of an Escherichia coli strain lacking sigma 32. A knockout mutation in rpoH1 did not abolish sigma 32 production in B. japonicum, and the rpoH1 mutant showed the wild-type growth phenotype, suggesting the presence of multiple rpoH homologs in this bacterium. Further characterization of the rpoH1 gene region revealed that the rpoH1 gene is located in a heat shock gene cluster together with the previously characterized groESL1 operon and three genes encoding small heat shock proteins in the following arrangement: groES1, groEL1, hspA, rpoH1, 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 groESL1 operon. Although the hspA-rpoH1 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.
Narberhaus
F
F
Mikrobiologisches Institut, Eidgenössische Technische Hochschule, Zürich, Switzerland. fnarber@micro.biol.ethz.ch
Weiglhofer
W
W
Fischer
H M
HM
Hennecke
H
H
D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics; D001483:Base Sequence; D018833:Chaperonins / Q000235:genetics; D003001:Cloning, Molecular; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D005816:Genetic Complementation Test; D006360:Heat-Shock Proteins / Q000235:genetics*; D018869:Heat-Shock Response / Q000235:genetics*; D006358:Hot Temperature; D008969:Molecular Sequence Data; D005810:Multigene Family; D009154:Mutation; D012230:Rhizobiaceae / Q000235:genetics*; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D012808:Sigma Factor / Q000235:genetics*; D014157:Transcription Factors; D014158:Transcription, Genetic
D001426:Bacterial Proteins; C097970:GroESL protein, Bacteria; D006360:Heat-Shock Proteins; D012808:Sigma Factor; D014157:Transcription Factors; C069774:heat-shock sigma factor 32; D018833:Chaperonins