Prolonged Stationary-Phase Incubation Selects for lrp Mutations in Escherichia coli K-12

Evolution by natural selection occurs in cultures ofEscherichia 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 ofrpoS , encoding the stationary-phase global regulator, ςS (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 anrpoS strain had acquired a GASP mutation inlrp . This GASP allele,lrp-1141 , encodes a mutant protein lacking the critical glycine in the turn of the helix-turn-helix DNA-binding domain. Thelrp-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 alrp null allele generated by a transposon insertion has a fitness gain identical to that of thelrp-1141 allele, verifying that cells lacking Lrp activity have a competitive advantage during prolonged starvation. Finally, we tested by genetic analysis the hypothesis that thelrp-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 thelrp GASP phenotype, and hence thelrp GASP phenotype is due to more global physiological changes.