Transcriptional control of the sulfur-regulated cysH operon, containing genes involved in L-cysteine biosynthesis in Bacillus subtilis

The molecular mechanisms of regulation of the genes involved in the biosynt hesis of cysteine are poorly characterized in Bacillus subtilis and other g ram-positive bacteria, In this study we describe the expression pattern of the B, subtilis cysH operon in response to sulfur starvation. A 6.1-kb poly cistronic transcript which includes the cysH, cysP, ylnB ylnC, ylnD, ylnE, and ylnF genes was identified. Its synthesis was induced by sulfur limitati on and strongly repressed by cysteine, The cysH operon contains a 5' leader portion homologous to that of the S box family of genes involved in sulfur metabolism, which are regulated by a transcription termination control sys tem, Here we show that induction of B, subtilis cysH operon expression is d ependent on the promoter and independent of the leader region terminator, i ndicating that the operon is regulated at the level of transcription initia tion rather than controlled at the level of premature termination of transc ription. Deletion of a 46-bp region adjacent to the -35 region of the cysH promoter led to high-level expression of the operon, el en in the presence of cysteine. We also found that O-acetyl-L-serine (OAS), a direct precursor of cysteine, renders cysH transcription independent of sulfur starvation a nd insensitive to cysteine repression. We propose that transcription of the cysH operon is negatively regulated by a transcriptional repressor whose a ctivity is controlled by the intracellular levels of GAS. Cysteine is predi cted to repress transcription by inhibiting the synthesis of GAS, which wou ld act as an inducer of cysH expression. These novel results provide the fi rst direct evidence that cysteine biosynthesis is controlled at a transcrip tional level by both negative and positive effecters in a gram-positive org anism.