The regulation of integrative recombination by the b2 region and the cII gene of bacteriophage λ

Abstract

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 λ int −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 λ cII − is defective in helping integrative recombination, λ b2 cII − 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 p I or p L promoters.