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J. Bacteriol., Aug 1995, 4364-4371, Vol 177, No. 15
FC Soncini, EG Vescovi and EA Groisman
The Salmonella typhimurium PhoP-PhoQ two-component regulatory system
controls the expression of several genes, some of which are necessary for
virulence. During a screening for PhoP-regulated genes, we identified the
phoPQ operon as a PhoP-activated locus. beta- Galactosidase activity
originating from phoPQ-lac transcriptional fusions required the presence of
both the transcriptional regulator PhoP and its cognate sensor-kinase PhoQ.
At low concentrations, PhoQ stimulated expression of phoPQ-lac
transcriptional fusions. However, larger amounts of PhoQ protein without a
concomitant increase in PhoP failed to activate phoPQ-lac fusions. Two
different transcripts are produced from the phoPQ operon during exponential
growth. These transcripts define two promoters: phoPp1, which requires both
PhoP and PhoQ for activity and which is environmentally regulated, and
phoPp2, which remains active in the absence of PhoP and PhoQ but which is
slightly stimulated by these proteins. The pattern of transcriptional
autoregulation was also observed at the protein level with anti-PhoP
antibodies. In sum, autoregulation of the phoPQ operon provides several
levels of control for the PhoP-PhoQ regulon. First, environmental signals
would stimulate PhoQ to phosphorylate the PhoP protein that is produced at
basal levels from the PhoP-PhoQ-independent promoter. Then, phospho-PhoP
would activate transcription of phoPp1, resulting in larger amounts of PhoP
and PhoQ and increased expression of PhoP- activated genes. A return to
basal levels could be mediated by a posttranscriptional mechanism by which
translation of the mRNA produced from phoPp1 is inhibited.
Copyright © 1995, American Society for Microbiology
Transcriptional autoregulation of the Salmonella typhimurium phoPQ operon
Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
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