Directed evolution of ionizing radiation resistance in Escherichia coli

Abstract

Four extremely radioresistant strains of Escherichia coli K12 were independently derived from MG1655 following repeated exposure to high dose ionizing radiation. D37 values for two of the strains approached that exhibited by Deinococcus radiodurans. Complete genome re‐sequencing using two different methods revealed that each strain acquired its radioresistance through very different sets of genetic changes. Nearly 70 different genetic alterations were present in each strain. The results do not reinforce any current idea about the mechanisms underlying the radiation resistance of species such as Deinococcus radiodurans. One mutation, a deletion of the e14 prophage, provides a partial explanation for the acquired phenotype. This is the only genome alteration that is common to all of the evolved strains. Additional mutations common to most cells in particular evolved populations were documented, but did not contribute measurably to radiation resistance when isolated in an otherwise wild type genetic background. Additional and different contributions to radiation resistance are represented in the genomic alterations in each of the three strains. In nature, multiple paths of evolutionary innovation can give rise to radioresistance even within a single species.