title: Asymmetric crossing over in the spontaneous formation of large deletions in the tonB-trp region of the Escherichia coli K-12 chromosome.
abstract: The chromosomal tonB gene of Escherichia coli was used as a target for the detection of spontaneous deletion mutations. The deletions were isolated in both recA+ and recA- cells, and mutants carrying large deletions were identified because they also lacked part or all of the trp operon. The frequencies of tonB-trp deletion were 1.79 x 10(-9) and 1.09 x 10(-9) for recA+ and recA- cells, respectively. We analyzed 12 deletions from recA+ and 10 from recA- cells by cloning and direct sequencing. The deletions ranged in size from 5612 bp to 15142 bp for recA+ and from 5428 bp to 13289 for recA- cells. Three deletions from recA+ cells and five deletions from recA- cells were found to have occurred between short sequence repeats at the termini of the deletion, leaving one copy of the repeat in the mutant sequence. Seven deletions from recA+ cells and three deletions from recA- cells did not have repeats at their termini; in these cases, the DNA sequences that are adjacent to the deletion termini in the wild-type are characterized by short (2-4 bp) repeats. From these results, a model is presented for the generation of deletion mutations which involves formation of an asymmetric crossover mediated by repeated sequences of 2- to 4-bp.
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics*; D001483:Base Sequence; D002876:Chromosomes, Bacterial / Q000235:genetics*; D003434:Crossing Over, Genetic; D004269:DNA, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics*; D029968:Escherichia coli Proteins; D005798:Genes, Bacterial; D008565:Membrane Proteins / Q000235:genetics*; D009154:Mutation; D009876:Operon; D011938:Rec A Recombinases / Q000235:genetics; D017384:Sequence Deletion; D014364:Tryptophan / Q000235:genetics