| Escherichia coli DNA polymerases II, IV and V serve dual roles within cells by facilitating efficient replication past potentially lethal DNA damage while simultaneously introducing genetic variation that can promote adaptation and evolution within stressful environments. While long recognized to be important for these physiological and evolutionary roles, the specific molecular mechanisms and relative contributions attributable to each of these alternative DNA polymerases within natural environmental conditions has remained elusive. Using a series of alternative polymerase-deficient strains analyzed during conditions of both feast and famine, we establish distinct hierarchies of polymerase activity. Pol II confers a significant physiological advantage by facilitating efficient replication and creating genetic diversity during periods of rapid growth, whereas Pol IV and Pol V make the largest contributions to evolutionary fitness during long-term stationary phase. Pol V is responsible for maximizing allelic diversity, yet Pol IV is the single greatest determinant of mutation frequency. Furthermore, we demonstrate that these alternative polymerases, along with additional members of the SOS regulon, are induced as cells transition from exponential to stationary phase growth in the absence of exogenous stress-stimulated SOS induction, and that they remain elevated throughout long-term stationary phase. These findings reveal each alternative DNA polymerase is vital to physiological and evolutionary fitness under dynamic and unpredictable conditions akin to those experienced in nature, and indicate their contributions to replication and adaptation within microbial communities are greater than previously appreciated. |
