Mutations that alter the DNA binding site for the bacteriophage lambda cII protein and affect the translation efficiency of the cII gene

Abstract

The efficiency of translation of the cII gene of bacteriophage lambda is greatly reduced by the cII3059 mutation, a GUU → GAU (Val → Asp) change in the second cII codon. Mutations in the third and fourth codons of the cII gene, called ctr mutations, reverse this translation deficiency. Lambda cII3059 ctr-1, which has a GCA å ACA (Ala å Thr) change in the fourth cII codon, produces about half the normal level of cII activity in liquid cultures, and λcII3059 ctr-2 and λcII3059 ctr-3, which have identical CGT å CGC changes in the third codon, produce normal levels of cII activity in liquid culture. Since the cII protein of λcII3059 ctr-3 has the same primary sequence as that of λcII3059, the cII − phenotype of λcII3059 can be explained entirely by the deficiency of translating cII mRNA. We propose that ctr mutations increase translation efficiency by destabilizing a stable stem structure which can be formed by cII mRNA. The ctr mutations lie in an overlapping regulatory region which contains, in addition to sequence elements that influence the rate of cII translation, a region to which cII protein binds to activate transcription from the P RE promoter. The ctr-1 mutation alters the cII recognition sequence from 5′-T-T-G-C-N 6T-T-G- C -3′ to 5′-T-T-G-C-N 6T-T-G- T -3′, but has no effect on P RE activity. Since a C å T change in the first (5′-proximal) T-T-G-C sequence (to yield 5′-T-T-G- T -N 6T-T-G-C) greatly lowers cII binding affinity, cII protein must not recognize the two T-T-G-C sequences in an identical manner.