Fig. 1

a: Cartoon representation of the canonical and “Pinholin-SAR endolysin” lysis paradigms of phages λ and ϕ21. The cartoon series begins with initial late gene expression and progresses downwards towards lysis as shown by the arrow. The cellular envelope components IM, PG and OM of a Gram-negative bacterial cell are shown as grey rectangle, hatched brown rectangle and black rectangle, respectively. In the canonical lysis paradigm (left), the holin (red ovals) accumulates in the IM while the active endolysin (open green symbols) accumulates in the cytoplasm. In the “Pinholin-SAR endolysin” lysis paradigm as in the phage ϕ21 (right), the SAR endolysins (closed green symbols) accumulate in the IM in an inactive form, anchored by a weakly hydrophobic TMD (green stub) alongside the pinholins (red ovals). See introduction for details on the pathways. b: Different spanin genetic architectures from the phages λ (embedded), P2 (overlapped), T4 (separated) and T1 (unimolecular). The rectangles, drawn to scale and labeled with appropriate names are color-coded to represent different lysis genes in each phage: red (holin), green (endolysin), blue (i-spanin), yellow (o-spanin) and cyan (u-spanin), while the uncolored rectangles represent genes of unrelated or unknown function. The spanin genes are also labeled with their spanin type on top of the gene in each case to highlight the genetic architecture. Note that color does not indicate sequence similarity; in fact, among all the genes depicted, only the K and R endolysin genes share any detectable similarity