Echos in Coliphage Secreted Endolysins

Sequence comprisons also reveal that a number of endo-lysins from phages of Gram-negative bacteria also have N-terminal sequences that could engage the sec system. There is a hydrophobic N-terminal extension on the predicted endolysins, all orthologs of T4 E, from coliphage P1 and from the lambdoid phages 21, P22, PS119 and PS3 (figure 10-6). The length of the hydrophobic sequence and the distribution of flanking charged residues are, however, indicative of a N-terminal TMD, a "signal anchor" or "uncleaved signal sequence,'' rather than of a cleavable signal peptide like the endolysin in fOg44. Experiments with clones of P1 lyz confirm that lysis, albeit somewhat delayed and less saltatory, follows after induction of this endolysin gene in the absence of a holin gene (118b). Moreover, cyanide treatment triggers early lysis and dramatically accelerates lysis already begun, whereas 1 mM azide, a specific inhibitor of SecA, blocks lysis. A detailed analysis of P1 Lyz confirmed that it is initially localized as an N-terminally tethered, enzymatically inactive form, and then is activated by release from the membrane (118b). This release occurs spontaneously at a low rate, thus accounting for the holin-independent lysis supported by lyz clone, or quantitatively, when the membrane is depolarized artificially or by the triggering of the P1 holin. The N-terminal TMD of Lyz was designated as a SAR

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