What makes the lysis clock tick? A study of the bacteriophage holin
Abstract
The timing of host lysis is the only decision made in the bacteriophage lytic
cycle. To optimize timing, double-stranded DNA phages use a 2-component lysis
system consisting of a muralytic enzyme, the endolysin, and a small membrane protein,
the holin, which controls the timing of lysis. The best characterized holin gene to date is
the S gene of bacteriophage λ.
One unusual feature of the S gene is that it produces two proteins of opposing
function: the holin, S105, and the antiholin, S107. Raab et al isolated and characterized
a number of S mutants, but all of them expressed both the holin and the antiholin; it is
possible, then, that the true extent of the holin-holin interactions were masked by
interactions with the antiholin. Thus, a large number of S105 mutants were created, and
their phenotypes characterized in the absence of the antiholin. The interaction between
those mutants and the wild-type were examined in an attempt to better understand what
determines the timing of hole formation by S105.
S105 and S107 differ only by two amino acids at the N-terminus; S107 has an
additional Met-Lys sequence. Previous studies have shown that S107 may have a different topology to S105, where the N-terminus of S107 is located in the cytoplasm
and is cannot flip through the membrane because of the extra cationic side chain. This
study investigates the role of the N-terminal transmembrane domain of the S proteins in
terms of hole formation and its role in the antiholin character of S107.
Previous results suggest that S105 forms hole via a large oligomeric structure
termed the “death raft”. The death raft model states that after S105 is inserted into the
membrane, it forms “rafts”, which grow in size until a spontaneous channel forms
leading to depolarization of the membrane and hole formation. This study investigates
the pathway of hole formation at the single-cell level, using a C-terminal fusion of S105
and green fluorescent protein, and attempts to address several of the predictions posed
by the death raft model.
Citation
White, Rebecca Lynn (2008). What makes the lysis clock tick? A study of the bacteriophage holin. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /ETD -TAMU -2766.