Endolysin Diversity

Four different enzyme activities have been associated with endolysins: ''true lysozyme'' (also known as muramidase; e.g.,T4 e lysozyme, P22 gp13 lysozyme), which hydrolyzes the 1,4 glycosidic bonds in the murein; transglycosylase (e.g., l R or P2 K), which attacks the same bonds but forms a cyclic 1,6-anhydro-N-acetylmuramic acid product; amidase (e.g.,T7 gp3.5), which hydrolyzes the amide bond between the N-aceytlmuramic acid and L-alanine residues in the oligopeptide crosslinking chains; and endopeptidase (e.g., f11 lysin), which attacks the peptide bonds in the same chains (79, 119, 121). In some cases, more than one of these activities is found in the same protein (79). Again, whenever the notion has been tested, with important exceptions noted below (The Other Paradigm: sec-Mediated Lysis), endolysins of different enzymatic types complement each other. Nonspecificity with respect to holins is clearly seen by comparing l and P22, which have nearly identical class I holins but completely different endolysins, and also P22 and PS34, which have nearly identical lysozymes orthologous to T4 E but unrelated class I and class II holins, respectively (figure 10-1A). Almost all endolysins are late proteins; a notable exception is T7 gp3.5, which is an early protein and has an important early function, as specific inhibitor of T7 RNA polymerase (34, 75). The holin-endolysin-RzRz1 lysis gene cassette conserved at the start of the late gene transcript of all lambdoid phages is present in the lateT7 transcript, except that the endolysin gene is replaced by an unrelated gene involved in phage DNA maturation (figure 10-1A).

Many endolysins of phages specific for Gram-positive hosts have a modular structure, with a C-terminal domain determining binding specificity exquisitely sensitive to the differences in the structure of the cell wall (37, 40, 41, 98). For example, the Cpl1 endolysin, a T4 lysozyme orthologue, from the pneumococcal phage Cp1 has a C-terminal domain specific for the choline component of the teichoic acid of its host, whereas the lysozyme from phage Cpl7, with a similar enzymatic domain, is choline-independent. Similarly, the Dp1 endolysin, an amidase, has a C-terminal cell wall binding domain orthologous to the choline-dependent domain of Cpll. The specificity and lytic activity have been exploited in the use of a phage endolysin as an aerosol to ablate pneumococcal infections in the mouse nasopharyngeal cavity, a development that highlights the possible use of phage proteins as antibacterial factors (80).

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