Dynamics of IS-related genetic rearrangements in resting Escherichia coli K-12

An analysis of restriction fragment length polymorphism (RFLP) using eightresidential insertion sequence (IS) elements as hybridization probesreveals that the genome of resting bacteria is more dynamic than it waslong believed. Escherichia coli strains stored in agar stabs for up to 30yr accumulate a genetic variation which is correlated to time of storage.This spontaneous mutagenesis is often IS-specific, with particularly highactivity for IS5, and thus suggests that transpositional DNA rearrangementsare a major cause for the observed genetic polymorphism. The RFLP patternsindicate a burst of IS30 transposition to occur occasionally. Mutation rateis estimated by two different methods to roughly 10(-5) IS-related DNArearrangements per bacterial chromosome per hour of storage for the eightIS elements studied. A pedigree derived from the RFLP data reveals thatpopulations had evolved independently in each stab and showed no signs ofconvergence. Relics of an assumed ancestral population were still presentin the stab cultures, but the elder stabs provided mostly mutants. Theseresults indicate that cells placed under nutritional deprivation might havea highly plastic genome and suggest that such plasticity might play anadaptive role.