Method for the isolation of Escherichia coli mutants with enhanced recombination between chromosomal duplications

A method is described for the isolation of Escherichia coli mutants that show increased recombination between a pair of chromosomal duplications. These "hyper-rec" mutants display a variety of secondary phenotypes. I have isolated a large number of hyper-rec mutants and found them useful in screening for mutants that accumulate labeled DNA fragments after short pulses with [3H]thymidine. The mutants so recovered include ones that are defective in deoxyribonucleic acid ligase, deoxyribonucleic acid polymerase I and its associated 5' yields 3' exonuclease, and a group of mutants, dnaS, that accumulate abnormally short Okazaki fragments. Evidence is presented that suggests that the lac-att80 segment of the chromosome cannot be inverted.