Transcriptional regulation of Bacillus subtilis citrate synthase genes

The Bacillus subtilis citrate synthase genes citA and citZ were repressed during early exponential growth phase in nutrient broth medium and were induced as cells reached the end of exponential phase. Both genes were also induced by treatment of cells with the drug decoyinine. After induction, the steady-state level of citZ mRNA was about five times higher than that of citA mRNA. At least some of the citZ transcripts read through into the isocitrate dehydrogenase (citC) gene. Transcription from an apparent promoter site located near the 3' end of the citZ gene also contributed to expression of citC. In minimal medium, citA transcription was about 6-fold lower when glucose was the sole carbon source than it was when succinate was the carbon source. Expression of the citZ gene was repressed 2-fold by glucose and 10-fold when glucose and glutamate were present simultaneously. This latter synergistic repression is similar to the effect of glucose and glutamate on steady-state citrate synthase enzyme activity. CitR, a protein of the LysR family, appeared to be a repressor of citA but not of citZ.