Studies of hyper-recombination and mutator strains of Escherichia Coli K12.

This project concerns hyper-recombination and mutator mutants of Escherichia coli K12 which have been obtained by an examination of colonies on BMB-lactose agar of an F-merodiploid strain, H2 lac⁻/F lac⁻, with different mutations in the lacZ genes, for those which showed an increased number of lac⁺ papillae.

Hyper-recombination mutants were a minority of the strains isolated by this method because the wild-type gene could only be formed by two cross-over events in a short segment of the chromosome; strains that were mutator or had duplications of a lacZ gene were frequent.

One of the hyper-recombination mutants, H2114, has been shown to carry a mutation, mapped on the Escherichia coli chromosome near thyA which in this strain and in other genetic backgrounds, increased the yield of P1- mediated transductants. The mutated gene in H2114 has been given the symbol, hypB, because it appeared to be different in phenotype and location from that described by Konrad and Lehman (1975)» In crosses with certain Hfr strains the yields of recombinants were higher than those obtained with the parent strains. The yield of recombinants from crosses in which strain H2114 acted as donor were also higher than those obtained with the parental F merodiploids. Recombination within short regions of the chromosome was examined by measuring the yields of ara⁺ recombinants in P1-mediated crosses with various ara⁻ donors. The relative enhancement of the yield was greater, the smaller the interval in which one cross-over event was obligatory. Recombination between mutants of phage λ was also increased. The enhanced recombination is dependent upon the recBC genes because recombination in double' mutants hypB recB was that expected of recB strains