Journal of Bacteriology, December 1998, p. 6283-6291, Vol. 180, No. 23
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Department of Biology, McMaster University, Hamilton, Ontario L8S 4K1, Canada
Received 8 June 1998/Accepted 24 September 1998
During entry into stationary phase, many free-living, gram-negative
bacteria express genes that impart cellular resistance to environmental
stresses, such as oxidative stress and osmotic stress. Many genes that
are required for stationary-phase adaptation are controlled by RpoS, a
conserved alternative sigma factor, whose expression is, in turn,
controlled by many factors. To better understand the numbers and types
of genes dependent upon RpoS, we employed a genetic screen to isolate
more than 100 independent RpoS-dependent gene fusions from a bank of
several thousand mutants harboring random, independent
promoter-lacZ operon fusion mutations. Dependence on RpoS
varied from 2-fold to over 100-fold. The expression of all fusion
mutations was normal in an rpoS/rpoS+
merodiploid (rpoS background transformed with an
rpoS-containing plasmid). Surprisingly, the expression of
many RpoS-dependent genes was growth phase dependent, albeit at lower
levels, even in an rpoS background, suggesting that other
growth-phase-dependent regulatory mechanisms, in addition to RpoS, may
control postexponential gene expression. These results are consistent
with the idea that many growth-phase-regulated functions in
Escherichia coli do not require RpoS for expression. The
identities of the 10 most highly RpoS-dependent fusions identified in
this study were determined by DNA sequence analysis. Three of the
mutations mapped to otsA, katE,
ecnB, and osmY
genes that have been previously
shown by others to be highly RpoS dependent. The six remaining
highly-RpoS-dependent fusion mutations were located in other genes,
namely, gabP, yhiUV, o371,
o381, f186, and o215.
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