Mutants of Escherichia coli K12 which affect excision of transposon Tn10.

We have described three illegitimate recombination events associated with, but not promoted by, transposon Tn10: precise excision, nearly precise excision, and precise excision of a nearly precise excision remnant. All three are structurally analogous: excision occurs between two short direct repeat sequences, removing all intervening material plus one copy of the direct repeat. In each case, the direct repeats border a larger inverted repeat. We report here the isolation of host mutants of Escherichia coli K12 which exhibit increased frequencies of precise excision of Tn10. Nineteen of the 39 mutants have been mapped to five distinct loci on the E. coli genetic map and have been designated texA through texE (for Tn10 excision). Mapping and genetic characterization indicate that each tex gene corresponds to a previously identified gene involved in cellular DNA metabolism: recB and/or recC, uvrD, mutH, mutS, and dam. The role of these various DNA repair and recombination genes in an illegitimate recombination process such as Tn10 excision will be discussed. In addition to an increase in precise excision frequency, all 39 tex mutants display an increased frequency for nearly precise excision. However, none of the mutants are increased for the third excision event, precise excision of a nearly precise excision remnant, supporting the idea that precise and nearly precise excision occur by closely related pathways which are distinct from those pathways which promote the third type of excision event.