Journal of Bacteriology, October 2000, p. 5885-5892, Vol. 182, No. 20
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Instituto de Biología Molecular y Celular de Rosario (IBR-CONICET) and Departamento de Microbiología, Facultad de Ciencias Bioquímicas y Farmacéuticas, Universidad Nacional de Rosario, Suipacha 531, 2000-Rosario, Argentina
Received 22 May 2000/Accepted 26 July 2000
The molecular mechanisms of regulation of the genes involved in the
biosynthesis of cysteine are poorly characterized in Bacillus subtilis and other gram-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 polycistronic
transcript which includes the cysH, cysP,
ylnB, ylnC, ylnD, ylnE,
and ylnF genes was identified. Its synthesis was induced by
sulfur limitation 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 system. Here we show
that induction of B. subtilis cysH operon expression is
dependent on the promoter and independent of the leader region
terminator, indicating that the operon is regulated at the level of
transcription initiation rather than controlled at the level of
premature termination of transcription. Deletion of a 46-bp region
adjacent to the
35 region of the cysH promoter led to
high-level expression of the operon, even 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 and insensitive to cysteine
repression. We propose that transcription of the cysH
operon is negatively regulated by a transcriptional repressor whose
activity is controlled by the intracellular levels of OAS. Cysteine is
predicted to repress transcription by inhibiting the synthesis of OAS,
which would act as an inducer of cysH expression. These
novel results provide the first direct evidence that cysteine
biosynthesis is controlled at a transcriptional level by both negative
and positive effectors in a gram-positive organism.
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