Inhibition of Bacillus subtilis aprE Expression by Lincomycin at the Posttranscriptional Level through Inhibition of ppGpp Synthesis

Abstract

Expression of the Bacillus subtilis alkaline protease gene aprE is controlled by many positive and negative regulators at the transcriptional level. During the course of screening for organic compounds that affect the expression of a translational aprE'-_??_lacZ fusion, we found that lincomycin (Lm), erythromycin and chloramphenicol exhibited an inhibitory effect in concentrations that hardly affected cell growth. The antibiotics are known to inhibit protein synthesis by binding to ribosomes. We chose one of them, Lm, for further study. We have previously shown that aprE expression requires guanosine 3', 5'-bisdiphosphate (ppGpp) synthesized on the ribosome by the stringent factor RelA. An examination of Lm-treated cells showed that the levels of ppGpp were greatly reduced in these cells, and the inhibitory effect of the antibiotic was not seen in relA-disruption mutants. Transcriptional levels of aprE, however, were not influenced by Lm treatment as shown by using a transcriptional aprE-lacZ fusion as well as quantitative RT PCR. Furthermore, disruption of relA did not affect the expression of transcriptional aprE-lacZ. From these results, we conclude that aprE expression is controlled by the stringent control at the posttranscriptional level, and that Lm inhibits this process by inhibiting ppGpp synthesis on the ribosome.