1.M.1 The Rz/Rz1 Spanin1 (Rz(1)) Family 

Bacteriophage lambda has four adjacent genes, S, R, Rz and Rz1, dedicated to host cell lysis. S encodes the holin and antiholin and R encodes the endolysin while Rz1 and Rz are an outer membrane lipoprotein (Rz1) and a type II signal anchor protein (Rz).  Together they comprise a spanin. The Rz-Rz1 complex spans the periplasm ane carries out the final step in the process of host lysis. Endolysin-mediated degradation of the cell wall is a prerequisite for conformational changes in the Rz-Rz1 complex leading to the juxtaposition and fusion of the two membranes. Fusion removes the last physical barrier to efficient release of progeny virions (Berry et al. 2008; Rajaure et al. 2015).  The lambda Rz and Rz1 lysis genes are remarkable in that Rz1, encoding the outer membrane lipoprotein, is completely embedded in the +1 register within Rz, which itself encodes the inner membrane protein (Summer et al. 2007; Berry et al. 2012; Berry et al. 2008; Kongari et al. 2018). 1.M.1.1 - 5 are most similar to the lambda phage i_spanin, 1.M.1.6 - 10 are derived from mycobacterial and gordonial phages and are most similar to each other, and 1.M.1.11 - 13 are derived from proteobacteria and are most similar to each other. These proteins are i-spanins. Only 1.M.1.1.1, 1.M.1.1.2 and 1.M.1.1.4 include both spanins (i-spanin and o-spanin) of the system.