Synthesis of two bacteriophage λ, S proteins in an in vivo system

Bacteriophage λ has two genes which are essential for lysis: R, a gene encoding a 158-amino-acid (aa) transglycosylase that attacks the peptidoglycanand S, a gene encoding two inner-membrane-associating proteins, designated S105 and S107 for their predicted lengths in aa residues. S105 and S107 are thought to have opposing roles in lysis, with the former acting as the lethal lysis effector and the latter as a lysis inhibitor. Here, we used a T7-polymerase-mediated expression system to show that S105 and S107 are synthesized at a constant ratio of about 2.5:1 throughout the period leading up to lysis, indicating that lysis scheduling does not require a translationally controlled switch from inhibitor (S107) to effector (S105) synthesis. However, evidence is presented that the mRNA sequences immediately 5' to the ribosome-binding site (RBS) of the S gene are required for the rather limited translation, but not the stability, of the S mRNA. No difference could be found in the pattern of ternary complex formation over the two S start codons in in vitro toe-printing assays with the wild-type mRNA and with mRNA deleted of the upstream sequences. Nevertheless, these results may suggest a role for translational control in S gene expression, if not in its temporal regulation or in the partition between S105 and S107 production.