Reexamining transcriptional regulation of the Bacillus subtilis htpX gene and the ykrK gene, encoding a novel type of transcriptional regulator, and redefining the YkrK operator.

HtpX is an integral cytoplasmic membrane metalloprotease well conserved in numerous bacteria. A recent study showed that expression of the Bacillus subtilis htpX gene is under dual negative control by Rok and a novel type of transcriptional regulator, YkrK. Here we report that expression of the B. subtilis htpX gene is strongly heat inducible. Contrary to the previous prediction, ykrK expression has been found to be not subject to autoregulation. We have identified the htpX promoter and the authentic ykrK promoter, which is also distinct from the previously predicted one. We have redefined a conserved inverted repeat sequence to be the YkrK operator, which is somewhat different from the previously proposed one. We provide evidence that YkrK is not a substrate of HtpX and that heat induction of htpX is not YkrK mediated. We have also found that the absence of FtsH or HtpX alone did not impair B. subtilis cell viability on LB agar plates at high temperature, whereas the absence of both FtsH and HtpX caused a severe growth defect under heat stress. This finding supports the notion that FtsH and HtpX may have partially overlapping functions in heat resistance. Finally, we show that htpX expression is subject to transient negative control by sigB under heat stress in a Rok- and YkrK-independent manner. Triple negative control of htpX expression at high temperature by rok, sigB, and ykrK may help cells to prevent uncontrolled and detrimental oversynthesis of the HtpX protease.