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 tra
nscripts, two of which are processed into three different products. We have
analysed the regulatory consequences of this complex transcriptional organ
ization in detail. First, the seven genes were heat induced to different ex
tents at the mRNA level and can be classified into three groups by their in
duction factors. This differential induction was also reflected at the prot
ein level. Secondly, the cellular amounts of the proteins HrcA, DnaK and Dn
aJ in B. subtilis differed drastically both under non-heat shock conditions
and after thermal upshock, Thirdly, Northern blot analyses demonstrated th
at an mRNA-processing reaction generating products of differential stabilit
ies plays an essential role during the regulation of gene expression, A cru
cial factor determining the low stability of two transcripts is the presenc
e 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 v
icinity of a Shine-Dalgarno sequence. These results show that B. subtilis i
s using various, especially post-transcriptional, regulatory mechanisms to
fine tune the expression of the individual genes of the heptacistronic dnaK
operon.