Bacteriophage λ Having Eco RI Endonuclease Sites Only in the Nonessential Region of the Genome

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Abstract
A derivative of λ b221 that has lost by mutation all EcoRI restriction sites has been isolated by alternative growth on restrictive and nonrestrictive strains. It has an efficiency of plating equal to 1 on the restrictive strain. Genetic cross of this bacteriophage with λ plac5 imm21 gave rise to recombinants of intermediate restricting ratios. The analysis of the EcoRI endonuclease-cleaved DNA by polyacrylamide gel electrophoresis, compared with the genetic results, has permitted identification of EcoRI endonuclease cleavage sites in the recombinants. The genotypes are: λ plac5 CI857 sRIλ30sRIλ20sRIλ10 and λ plac5 CI857 sRIλ20sRIλ10. The remaining cleavage sites, respectively, sRIlac sRIλ4 and sRIlac sRIλ4 sRIλ3, are all located in a region nonessential for bacteriophage multiplication. The involvement of these mutant bacteriophages as vector for foreign genes are discussed.