DNA degradation in the terminus region of resolvase mutants of Escherichiacoli, and suppression of this degradation and the Dif phenotype by recD

We recently proposed that guillotining of dimer chromosomes occurs at cell division in resolvase mutants of Escherichia coli. This was based on the ab normal pattern of cell division observed in 10-14% of the cells in microcol onies of xerC, xerD and dif mutants. A prediction of this guillotining is t hat DNA degradation should occur in the terminus region, in the vicinity of the dif locus.. We have tested this by DNA-DNA hybridization and have obse rved that dif was absent in about 22% of the chromosomes in exponentially g rowing xerC mutants. A locus 206 kb from nif was not affected by this degra dation We-have also observed that degradation did not occur in xerC recD mu tants, and that the low efficiency of plating associated with the Dif pheno type was suppressed in this strain. A model is proposed in which rapid degr adation of the terminus region does not occur in recD mutants following gui llotining, and that this permits the initiation of repair of broken dimer c hromosomes prior to completion of cell division. (C) 2001 Societe francaise de biochimie et biologie moleculaire / Editions scientifiques et medicales Elsevier SAS.