Transcriptional and posttranscriptional control of the Bacillus subtilis succinate dehydrogenase operon

The amount of succinate dehydrogenase (SDH) in Bacillus subtilis varies with growth conditions. In this work we studied the steady-state level and the rate of decay of B. subtilis sdh mRNA under different growth conditions. In exponentially growing cells, the steady-state level of sdh mRNA was severalfold lower when glucose was present compared with growth without glucose, whereas the rate of decay of sdh mRNA was the same with and without glucose. Thus, glucose repression seems to act by decreasing sdh mRNA synthesis. When the bacteria entered the stationary phase, the steady-state level of sdh mRNA dropped about sixfold. At the same time, sdh mRNA half-life decreased from 2.6 to 0.4 min. This result indicates that transcription of the sdh operon is initiated at the same rate in exponentially growing and in stationary-phase cells. The start point of the sdh transcripts, as measured by primer extension, was the same under all conditions studied, suggesting that the sdh operon is solely controlled by the previously identified sigma 43-like promoter. The increase of SDH activity in stationary phase may be explained by reduced dilution of the SDH proteins as a result of the retarded growth rate. We suggest that enhanced degradation of the sdh transcript is a means by which the bacteria adjust expression to the demands of stationary phase.