Role of homology and pathway specificity for recombination between plasmids and bacteriophage λ

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Abstract
To determine the minimum amount of homology required for efficient recombination in Escherichia coli, we measured recombination frequencies between bacteriophage λ and pBR322 derivatives containing λ DNA fragments of various sizes by assaying for phages that could transduce the bla and ori genes of pBR322. Efficient recombination required about 40 bp of homology; increases in homology above 40 bp resulted in proportionate increases in recombination, while decreases below 40 bp resulted in precipitous decreases in recombination. The recA + gene stimulated recombination over the entire range of homologies tested. Restriction enzyme digests of several recombinant DNA molecules indicated that they contained the complete plasmid DNA inserted in the λ genome as expected for a reciprocal crossover. Analysis of recombination frequencies in different recombination-deficient mutant strains indicated that the formation of λ-plasmid cointegrates by homologous recombination proceeded predominantly by the RecBC pathway and very inefficiently, if at all, by the RecE and RecF pathways.