Prolonged stationary-phase incubation selects for lrp mutations in Escherichia coli K-12

Evolution by natural selection occurs in cultures of Escherichia coli maint ained under carbon starvation stress. Mutants of increased fitness express a growth advantage in stationary phase (GASP) phenotype, enabling them to g row and displace the parent as the majority population. The first GASP muta tion was identified as a loss-of-function allele of rpoS, encoding the stat ionary-phase global regulator, sigma(S) (M. M. Zambrano, D. A. Siegele, M. A. Almiron, A. Tormo, and R. Kelter, Science 259:1757-1760, 1993). We now r eport that a second global regulator, Lrp, can also play a role in stationa ry-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, l rp-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 overexpre ssed. Hence, the mutant protein appears to retain stability and the ability to dimerize but lacks DNA-binding activity. We also demonstrated that a Ip p null allele generated by a transposon insertion has a fitness gain identi cal to that of the lrp-1141 allele, verifying that cells lacking Lrp activi ty have a competitive advantage during prolonged starvation. Finally, we te sted by genetic analysis the hypothesis that the lrp-1141 GASP mutation con fers a fitness gain by enhancing amino acid catabolism during carbon starva tion. 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 d ue to more global physiological changes.