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Description:
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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 . |