SITE CONTROLLING THE SPECIFICITY OF N-ACTION IS OUTSIDE THE PROMOTER-OPERATOR REGION - TRIPLE HYBRID PHAGE N21-LAMBDA IMM434NIN5

A short interval of homology between immλ, imm434 and imm21 DNAs was identified near the leftward promoter-operator region. This homology, denoted Hs, was revealed by electron microscopic examination of λimmλ/λimm21 and λimm434/λimm21 heteroduplexes, and permitted us to construct a special λ hybrid (λhyB) which contains the N region of phage 21 and the adjacent imm region from phage 434. This triple hybrid, λN21imm434nin5, was analysed by genetic, transcriptional and electronic micrographic techniques. Its leftward and rightward promoter-operator regions are of phage 434 specificity and are controlled by the 434 repressor. Suprisingly, the N21 gene of λhyB was found to be defective, perhaps to preserve the viability of the hybrid. Its leftward N-recognition system (nutL) is of phage 21 specificity since it responds only to the N21 function in complementation tests, as measured by antitermination of leftward transcription initiated at the pL promotor in the imm434 region. We conclude, therefore, that the pLoL region of 434 contains no information for the specificity of N antitermination. Both λimm21 and λhyB were found to be missing the tL1 terminator function (see also Salstrom and Szybalski, 1978b). In these phages, the tL2 terminator was found to be only 60% effective under N-21 conditions, and therefore expression of their red-gam genes is sufficient to endow the λhyB and λN-21imm21nin5 phages with the Fec+ phenotype. © 1979.