Asymmetric crossing over in the spontaneous formation of large deletions in the tonB-trp region of the Escherichia coli K-12 chromosome

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 freq uencies 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(+) a nd 10 from recA(-) cells by cloning and direct sequencing. The deletions ra nged 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 f rom recA(-) cells were found to have occurred between short sequence repeat s at the termini of the deletion, leaving one copy of the repeat in the mut ant sequence. Seven deletions from recA(+) cells and three deletions from r ecA(-) 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 c haracterized by short (2-4 bp) repeats. From these results, a model is pres ented for the generation of deletion mutations which involves formation of an asymmetric crossover mediated by repeated sequences of 2- to 4-bp.