REGULATION OF INTEGRATIVE RECOMBINATION BY THE B2 REGION AND THE CLL GENE OF BACTERIOPHAGE-LAMBDAL

We have used a PP′ × BB′ prophage excision system to study the regulation of int expression in bacteriophage λ. The assay is based on heteroimmune infection of a lysogen [carrying a λint-gal prophage flanked by the phage (PP′) and the bacterial (BB′) attachment sites] with an int+ helper. The Int function provided by the helper excises the gal prophage, which subsequently is detected in the lysate. This Int-promoted prophage excision depends on int+ and cII+ genes, is independent of Rec and Red systems, is thermolabile, and depends on a high multiplicity of infection with the cII+ helper, in agreement with known characteristics of integrative recombination. While λcII- is defective in helping integrative recombination, λb2cII- is proficient in promoting the reaction. A cis-trans complementation test shows that the inhibitor in the b2 region acts in cis to the active int gene. These results suggest that the b2 region contains a nondiffusible inhibitor of integration which is overcome by the cII gene product. We discuss the possible role of a b2 inhibitory site on Int expression from pI or pL promoters. © 1979.