1.E.1 The P21 Holin S (P21 Holin) Family
Phage 21 S holin is the prototype for class II holins. It has two TMSs with both the N- and C-termini on the cytoplasmic side of the inner membrane of E. coli (Gründling et al. 2000). As for other holins, it functions in the export of an endolysin, but the holin channel also allows release of small ions and metabolites (Park et al. 2006). TMS1 may be dispensable for function.
A homologue of the P21 holin is the holin of bacteriophage H-19B. The gene encoding it has been associated with the Shiga-like Toxin I gene in E. coli (Neely and Friedman, 1998). It may function in toxin export as has been proposed for the X. nematophila holin-1 (Brillard et al., 2003; TC #1.E.2.1.4).