DYNAMICS OF IS-RELATED GENETIC REARRANGEMENTS IN RESTING ESCHERICHIA-COLI K-12

An analysis of restriction fragment length polymorphism (RFLP) using e ight residential insertion sequence (IS) elements as hybridization pro bes reveals that the genome of resting bacteria is more dynamic than i t was long believed. Escherichia coli strains stored in agar stabs for up to 30 yr accumulate a genetic variation which is correlated to tim e of storage. This spontaneous mutagenesis is often IS-specific, with particularly high activity for IS5, and thus suggests that transpositi onal DNA rearrangements are a major cause for the observed genetic pol ymorphism, The RFLP patterns indicate a burst of IS30 transposition to occur occasionally. Mutation rate is estimated by two different metho ds to roughly 10(-5) IS-related DNA rearrangements per bacterial chrom osome per hour of storage for the eight IS elements studied. A pedigre e derived from the RFLP data reveals that populations had evolved inde pendently in each stab and showed no signs of convergence. Relics of a n assumed ancestral population were still present in the stab cultures , but the elder stabs provided mostly mutants. These results indicate that cells placed under nutritional deprivation might have a highly pl astic genome and suggest that such plasticity might play an adaptive r ole.