Growth-dependent and adaptive mutation rates to ebgR and IS30 transposition in the bacteria Escherichia coli K-12 at different extracellular Mg2+ concentrations

Abstract

During starvation on carbon and energy Escherichia coli K-12 cells, modified to possess EbgA51 as the only く-galactosidase enzyme, experience adaptive mutations in the ebgR repressor gene. In this way, cells acquire the capacity to utilize the lactulose as the only source of carbon and energy and begin to grow. Adaptive mutations at ebgR are mediated largely by insertion sequences, 40% of adaptive mutants contain IS30 insertions. Also, besides sensing extracellular Mg2+, a PhoP-PhoQ system decreases the adaptive mutation rate to ebgR in a to-date unknown way. By performing fluctuation tests and genetic analyses, we tested the hypothesis that Mg2+ plays an important role in the adaptive mutation at ebgR. Results gathered with phoP and phoQ mutant strains demonstrated that the adaptive, but not the growth-dependent, mutation rate is increased by a high extracellular Mg2+ concentration. In an Mg2+-rich environment, the phoQ cells experience a nearly identical adaptive mutation rate as the wild-type strain. Results with the wild-type strain show that the relation between the levels of PhoP-PhoQ expression and the adaptive mutation rate is not as straightforward as expected and that different Mg2+ concentrations do not affect IS30 transposition. We discuss the possible role of magnesium in the adaptive mutation process.