Phage holins and lytic enzymes are both necessary for bacterial lysis and virus dissemination. This family also includes TcdE/UtxA involved in toxin secretion in Clostridium difficile [1]. The 1.E.10 family is represented by Bacillus subtilis phi29 holin [2,3]; 1.E.16 represents the Cph1 holin[4]; and the 1.E.19 family is represented by the Clostridium difficile TcdE holin. Toxigenic strains of C. difficile produce two large toxins (TcdA and TcdB) encoded within a pathogenicity locus. tcdE, encoded between tcdA and tcdB, encodes a 166 aa protein which causes death to E. coli when expressed, and the structure of TcdE resembles holins. TcdE acts on the bacterial membrane. Since TcdA and TcdB lack signal peptides, they may be released via TcdE either prior to or subsequent to cell lysis [1].

1: Evidence for holin function of tcdE gene in the pathogenicity of Clostridium difficile. Tan KS, Wee BY, Song KP; J Med Microbiol 2001;50:613-619. PMID:11444771

2: The missing link in phage lysis of gram-positive bacteria: gene 14 of Bacillus subtilis phage phi 29 encodes the functional homolog of lambda S protein. Steiner M, Lubitz W, Blasi U; J Bacteriol. 1993;175:1038-1042. PMID:8432697

3: Characterization and determination of holin protein of Streptococcus suis bacteriophage SMP in heterologous host. Shi Y, Yan Y, Ji W, Du B, Meng X, Wang H, Sun J; Virol J. 2012;9:70. PMID:22436471

4: Functional analysis of the two-gene lysis system of the pneumococcal phage Cp-1 in homologous and heterologous host cells. Martin AC, Lopez R, Garcia P; J Bacteriol. 1998;180:210-217. PMID:9440507