The site controlling the specificity of N action is outside the promoter-operator region: a triple hybrid phage lambda N21 imm434nin5

Abstract

A short interval of homology between imm lambda, imm434 and imm21 DNAs was identified near the leftward promoter-operator region. This homology, denoted Hs, was revealed by electron microscopic examination of lambda imm lambda/lambda imm21 and lambda imm434/lambda imm21 heteroduplexes, and permitted us to construct a special lambda hybrid (lambda hyB) which contains the N region of phage 21 and the adjacent imm region from phage 434. This triple hybrid, labmda N21 imm434nin5, 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. Surprisingly, the N21 gene of lambda hyB 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 lambda imm21 and lambda hyB 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 N21 conditions, and therefore expression of their red-gam genes is sufficient to endow the lambda hyB and lambda N21- imm21nin5 phages with the Fec+ phenotype.