Expression of the Bacillus subtilis sulphonate-sulphur utilization genes is regulated at the levels of transcription initiation and termination

The Bacillus subtilis ssuBACD gene cluster is required for the utilization of aliphatic sulphonates as sulphur sources. The regulation of expression of the ssu genes was studied in constructs carrying chromosomal transcriptional fusions of the ssu promoter region to lacZ. When sulphate or cystine served as sulphur sources, expression of the ssu genes was repressed. A putative terminator located between the promoter and the start of the ssuB gene partially overlaps a putative antiterminator. Removal of both the antiterminator and the terminator led to a decrease in the repression ratio of about 10-fold, but not to constitutive expression. Replacement of the ssu promoter by the Pspac promoter led to decreased expression of the ssu genes, but not to loss of repression by sulphate and cystine. Thus, the repression exerted by sulphate and cystine resulted from regulation at the level of both transcription initiation and transcription termination. O-acetyl-L-serine, a precursor of cysteine, served as effector molecule in both regulation systems.