Promotion of Evolution by Intracellular Coexistence of Mutator and Normal DNA Polymerases

The efficient evolution of a population requires both genetic diversity and stable reproduction of advantageous genotypes. The accuracy of DNA replication guarantees the stable reproduction, while errors during DNA replication produce the genetic diversity. Thus, one key to the promotion of evolution is inherent in DNA replication. In bacteria, replication forks progress bidirectionally from the single origin of replication on a genome. One replication fork contains two DNA polymerase molecules so that four DNA polymerases simultaneously carry out the replication of a genome. It is generally believed that the fidelity of the intracellular DNA polymerases is identical (parity strategy). To test this, we examined the effects of the intracellular coexistence of a mutator polymerase with low fidelity and a normal polymerase with high fidelity on adaptive evolution (disparity strategy). From the analysis using genetic algorithms based on the bacterial replication, it was found that the population using the disparity strategy could further expand its genetic diversity and preserve the advantageous genotypes more profoundly than the parity population. This strongly suggests that bacteria replicating with a disparity strategy may undergo rapid evolution, particularly during severe environmental changes. The implications of the conspicuous adaptability of Escherichia coli mutator strains are discussed in this context.