THE LETHAL LAMBDA-S GENE ENCODES ITS OWN INHIBITOR

The 107 codon reading frame of the λ lysis gene S begins with the codon sequence Met1-Lys2-Met3..., and it has been demonstrated in vitro that both Met codons are used for translational starts. Furthermore, the partition of initiation events at the two start codons strongly affects the scheduling of lysis. We have presented a model in which the longer product, S107, acts as an inhibitor of the shorter product, S105, the lethal lysis effector, despite the fact that the two molecules differ only in the Met-Lys residues at the amino terminus of S107. Using immunological and biochemical methods, we show in this report that the two predicted protein products, S105 and S107, are detectable in vivo as stable, membrane-bound molecules. We show that S107 acts as an inhibitor in trans, and that its inhibitory function is entirely defined by the positively charged Lys2 residue. Moreover, our data show that energy poisons abolish the inhibitory function of S107 and simultaneously convert S107 into a lysis effector. We propose a two step model for the lethal action of gene S: first, induction of the S gene results in the accumulation of S105 and S107 molecules in mixed oligomeric patches in the cytoplasmic membrane; second, S monomers rearrange by lateral diffusion within the patch to form an aqueous pore. The R gene product, a transglycosylase, is released through the pore to the periplasm, resulting in destruction of the peptidoglycan and bursting of the cell. According to this model, the lateral diffusion step is inhibited by the energized state of the membrane. A simple basis for this inhibition could be the ionic interaction between the negatively charged inner surface of the cytoplasmic membrane and the positively charged Lys2 residue of S107. Dissipation of the membrane potential by formation of a functional pore abolishes this interaction and allows rearrangement of S105/S107 hetero-oligomers. Thus, not only do the two protein products of the S gene have opposing function but S107 itself has a dual capacity. S107 acts as a lysis inhibitor as long as the infected cell can maintain the membrane potential, and it also contributes actively to pore formation when the membrane potential collapses, i.e. once a single lysis pore has been formed. Hence, the function of the S gene is inherently saltatory, so that progeny phage are efficiently released in the lytic burst.