1.E.64.  The P7 Phage Holin LydD (LydD) Family

P1 and P7 are model temperate myoviruses. They infect different Enterobacteriaceae and can develop lytically or form lysogens. Only some P1 adaptation strategies to propagate in different hosts are known. An atypical feature of P1 is the number and organization of cell lysis-associated genes. In addition to SAR-endolysin Lyz, holin LydA, and antiholin LydB, P1 encodes other predicted holins, LydC and LydD (Bednarek et al. 2022). LydD is encoded within the same operon as Lyz; LydA and LydB are encoded withiin an unlinked operon, and LydC is encoded within an operon preceding the lydA gene. By analyzing the phenotypes of P1 mutants in known or predicted holin genes, Bednarek et al. 2022 showed that all of the products of these genes cooperate with the P1 SAR-endolysin in cell lysis and that LydD is a pinholin. The contributions of holins/pinholins to cell lysis by P1 appear to vary depending on the host of P1 and the bacterial growth conditions. The pattern of morphological transitions characteristic of SAR-endolysin-pinholin action dominates during lysis by wild-type P1, but in the case of a lydC lydD mutant, it changes to that characteristic of classical endolysin-pinholin action. The complex lytic system facilitates P1 adaptation to various hosts and their growth conditions (Bednarek et al. 2022).