The dnaKJ operon belongs to the σ³²-dependent class of heat shock genes in Bradyrhizobium japonicum

Abstract

The dnaKJ genes of Bradyrhizobium japonicum were cloned and sequenced. They map adjacent to each other, as in other proteobacteria of the α and γ subgroups. Primer extension experiments identified two strongly heat-inducible transcripts starting 99 bp (T1) and 204 bp (T2) upstream of dnaK. Synthesis of the shorter transcript T1 in Escherichia coli required the presence of a recently characterized σ³² homologue (RpoH₁) from B. japonicum. The −35 and −10 regions of the promoters associated with the transcription start sites T1 and T2 displayed nucleotide sequence motifs that are characteristic for σ³²-dependent promoters in E.␣coli and α-proteobacteria. Heat shock regulation of dnaK expression was confirmed by immunoblot analysis of DnaK protein. All of these results put dnaK into the σ³²-dependent class, not the CIRCE-dependent class, of heat shock genes in B. japonicum. At normal growth temperature dnaK was expressed at a significant basal level. All attempts to eliminate dnaK function by insertion or deletion mutagenesis failed. By contrast, dnaJ null mutants and insertions in the dnaKJ intergenic region were easily obtained. The growth rate of dnaJ mutants was reduced but the final cell density reached in rich medium and their symbiotic properties were indistinguishable from the wild type.