Competitive Fitness during Feast and Famine: How SOS DNA Polymerases Influence Physiology and Evolution in Escherichia coli.

Escherichia coli DNA polymerases II, IV and V serve dual roles by facilitating efficient replication past DNA damage while simultaneously introducing genetic variation that can promote adaptive evolution. Here we show that these alternative polymerases are induced as cells transition from exponential to long-term stationary phase growth in the absence of SOS induction with external agents. By monitoring the relative fitness of isogenic mutant strains expressing only one alternative polymerase over time, spanning hours to weeks, we establish distinct growth phase-dependent hierarchies of polymerase mutant strain competitiveness. Pol II confers a significant physiological advantage by facilitating efficient replication and creating genetic diversity during periods of rapid growth. Pol IV and Pol V make the largest contributions to evolutionary fitness during long-term stationary phase. Consistent with their roles providing both a physiological and adaptive advantage during stationary phase, the expression patterns of all three SOS polymerases changes during the transition from log phase to long-term stationary phase. Compared to the alternative polymerases, Pol III transcription dominates during mid-exponential phase, however its abundance decreases to less than 20% during long-term stationary phase. Pol IV transcription dominates as cells transition out of exponential phase into stationary phase and a burst of Pol V transcription is observed as cells transition from death phase to long-term stationary phase. These changes in alternative DNA polymerase transcription occur in the absence of SOS induction by exogenous agents and indicate that cell populations require appropriate expression of all three alternative DNA polymerase during exponential, stationary and long-term stationary phases to attain optimal fitness and undergo adaptive evolution.