A host recognition module shared among distant Alteromonas bacteriophage families features tail fibers with transient chaperone caps

The host recognition modules encoding the injection machinery and receptor binding proteins (RBPs) of bacteriophages are variable genomic units predisposed to mutation and recombination to maintain infectivity toward co-evolving bacterial hosts. In this study, we reveal how Alteromonas mediterranea schitovirus A5 shares its host recognition module, including tail fiber (TF) and cognate chaperone, with phages from distantly related families including Alteromonas myovirus, V22. While the chaperone of V22 is essential for producing active TFs, here we demonstrate production of functional A5 TFs regardless of chaperone co-expression. AlphaFold-generated models of TF and chaperone pairs from phages A5, V22, and other Alteromonas phages reveal how amino acid insertions within both A5-like proteins results in a knob domain duplication in the TF and a {beta}-hairpin "tentacle" extension of the chaperone. These structural modifications are linked to chaperone dependency differences between the A5 and V22 TFs. Structural similarity between the chaperones and intramolecular chaperone domains of other phage RBPs suggests an additional function of these chaperones as transient TF "caps". Finally, our identification of homologous host recognition modules used by morphologically distinct phages implies that HGT and recombination events between unrelated phages may be a more common process than previously thought among Caudoviricetes phages.