Post-transcriptional regulation of the Bacillus subtilis dnaK operon

The heptacistronic dnaK heat shock operon of Bacillus subtilis consists of the genes hrcA, grpE, dnaK, dnaJ, orf35, orf28 and orf50. It is controlled by the CIRCE/HrcA operator/repressor system and specifies three primary transcripts, two of which are processed into three different products. We have analysed the regulatory consequences of this complex transcriptional organization in detail. First, the seven genes were heat induced to different extents at the mRNA level and can be classified into three groups by their induction factors. This differential induction was also reflected at the protein level. Secondly, the cellular amounts of the proteins HrcA, DnaK and DnaJ in B. subtilis differed drastically both under non-heat shock conditions and after thermal upshock. Thirdly, Northern blot analyses demonstrated that an mRNA-processing reaction generating products of differential stabilities plays an essential role during the regulation of gene expression. A crucial factor determining the low stability of two transcripts is the presence of the CIRCE element at their 5' ends. We demonstrate that CIRCE leads to the destabilization of mRNAs, but only if it is located in the immediate vicinity of a Shine-Dalgarno sequence. These results show that B. subtilis is using various, especially post-transcriptional, regulatory mechanisms to fine tune the expression of the individual genes of the heptacistronic dnaK operon.