THOMAS et al.1 have constructed a mutant strain of bacteriophage λ (λgt.λC) which is especially well suited to studies involving the cloning of recombinant DNA molecules. Specifically, this bacteriophage, deleted of 15% of its genome, has only two EcoRI restriction endonuclease sites which, when cleaved, reduce the phage chromosome to three DNA fragments. The larger of these, the right- and left-hand arms of the chromosome, encode all functions necessary for lytic infection. The smaller, central fragment consisting of approximately 5.000 base pairs encodes the λ attachment site, the Int and Xis functions and a necessary part of the λ-generalised recombination system, but no genes necessary for phage propagation. Although left- and right-hand restriction fragments have all necessary λ genes, they seem to lack sufficient length of DNA for packaging into a viable phage particle1. This provides a powerful positive selection for recombinant DNA molecules consisting of isolated and rejoined right and left restriction fragments which have also incorporated a fragment of foreign DNA restoring the λ chromosome to viable size.
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