Escherichia coli incubated in phosphate-limiting minimal medium dies during prolonged incubation as a result of the production of acetic acid. Variants that consume acetic acid generally sweep through the population after three serial cultures. Evolvability may primarily result from induction of the potentially mutagenic LexA DNA damage response or from growth of preexisting mutants. Cells starved of phosphate induce the LexA regulon through a unique mechanism based on an increase in the internal pH at the approach of the stationary phase. Evolved cells resume growth on phosphorylated products as a result of the activation of the cryptic PhnE permease. Here, it is shown that first PhnE-expressing revertants swept through starved populations independently of the expression of the LexA regulon. Induction of the LexA regulon and especially of the translesion synthesis DNA polymerases Pol IV and Pol V was, however, absolutely required for the ultimate evolution of acetic acid-detoxifying mutant strains. Both growth under selection and induction of translesion synthesis DNA polymerases are therefore required for adaptive evolution under phosphate starvation conditions.

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