Journal of Bacteriology, August 2000, p. 4361-4365, Vol. 182, No. 15
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, Massachusetts 02115
Received 30 March 2000/Accepted 11 May 2000
Evolution by natural selection occurs in cultures of
Escherichia coli maintained under carbon starvation stress.
Mutants of increased fitness express a growth advantage in stationary
phase (GASP) phenotype, enabling them to grow and displace the parent as the majority population. The first GASP mutation was identified as a
loss-of-function allele of rpoS, encoding the
stationary-phase global regulator,
S (M. M. Zambrano, D. A. Siegele, M. A. Almirón, A. Tormo, and R. Kolter, Science 259:1757-1760, 1993). We now report that a second
global regulator, Lrp, can also play a role in stationary-phase competition. We found that a mutant that took over an aged culture of
an rpoS strain had acquired a GASP mutation in
lrp. This GASP allele, lrp-1141, encodes a
mutant protein lacking the critical glycine in the turn of the
helix-turn-helix DNA-binding domain. The lrp-1141 allele
behaves as a null mutation when in single copy and is dominant negative
when overexpressed. Hence, the mutant protein appears to retain
stability and the ability to dimerize but lacks DNA-binding activity.
We also demonstrated that a lrp null allele generated by a
transposon insertion has a fitness gain identical to that of the
lrp-1141 allele, verifying that cells lacking Lrp activity
have a competitive advantage during prolonged starvation. Finally, we
tested by genetic analysis the hypothesis that the lrp-1141
GASP mutation confers a fitness gain by enhancing amino acid catabolism
during carbon starvation. We found that while amino acid catabolism may
play a role, it is not necessary for the lrp GASP
phenotype, and hence the lrp GASP phenotype is due to more
global physiological changes.
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