Bacteriophage lambda having EcoRI endonuclease sites only in the nonessential region of the genome

A derivative of lambda 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 lambda 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: lambda plac5 CI857 sRIlambda3(0)sRIlambda2(0)sRIlambda1(0) and lambda plac5 CI857 sRIlambda2(0)sRIlambda1(0). The remaining cleavage sites, respectively, sRIlac sRIlambda4 and sRIlac sRIlambda4 sRIlambda3, are all located in a region nonessential for bacteriophage multiplication. The involvement of these mutant bacteriophages as vector for foreign genes are discussed.