Dual translational initiation sites control function of the lambda S gene

Lysis gene S of phage lambda has a 107 codon reading frame beginning with the codons Met1‐Lys2‐Met3. Genetic data have suggested that translational initiation occurs at both Met1 and Met3, generating two polypeptides, S107 and S105 respectively. We have proposed a model in which the proper scheduling of lysis depends on the partition of translational initiations between the two start codons. Here, using in vitro methods, we show that two stem‐loop structures, one immediately upstream of the reading frame and a second approximately 10 codons within the gene, control the partitioning event. Utilizing primer‐extension inhibition or ‘toeprinting’, we show that the two S start codons are served by two adjacent Shine‐Dalgarno sequences. Moreover, the timing of lysis supported by the wild‐type and a number of mutant alleles in vivo can be correlated with the ratio of ternary complex formation over Met1 and Met3 in vitro. Thus the regulation of the S gene is unique in that the products of two adjacent in‐frame initiation events have opposing function.