DNA polymerase swapping in Caudoviricetes bacteriophages.

backgroundViruses with double-stranded (ds) DNA genomes in the realm Duplodnaviria share a conserved structural gene module but show a broad range of variation in their repertoires of DNA replication proteins. Some of the duplodnaviruses encode (nearly) complete replication systems whereas others lack (almost) all genes required for replication, relying on the host replication machinery. DNA polymerases (DNAPs) comprise the centerpiece of the DNA replication apparatus. The replicative DNAPs are classified into 4 unrelated or distantly related families (A-D), with the protein structures and sequences within each family being, generally, highly conserved. More than half of the duplodnaviruses encode a DNAP of family A, B or C. We showed previously that multiple pairs of closely related viruses in the order Crassvirales encode DNAPs of different families.

methodsGroups of phages in which DNAP swapping likely occurred were identified as subtrees of a defined depth in a comprehensive evolutionary tree of tailed bacteriophages that included phages with DNAPs of different families. The DNAP swaps were validated by constrained tree analysis that was performed on phylogenetic tree of large terminase subunits, and the phage genomes encoding swapped DNAPs were aligned using Mauve. The structures of the discovered unusual DNAPs were predicted using AlphaFold2.

resultsWe identified four additional groups of tailed phages in the class Caudoviricetes in which the DNAPs apparently were swapped on multiple occasions, with replacements occurring both between families A and B, or A and C, or between distinct subfamilies within the same family. The DNAP swapping always occurs "in situ", without changes in the organization of the surrounding genes. In several cases, the DNAP gene is the only region of substantial divergence between closely related phage genomes, whereas in others, the swap apparently involved neighboring genes encoding other proteins involved in phage genome replication. In addition, we identified two previously undetected, highly divergent groups of family A DNAPs that are encoded in some phage genomes along with the main DNAP implicated in genome replication.

conclusionsReplacement of the DNAP gene by one encoding a DNAP of a different family occurred on many independent occasions during the evolution of different families of tailed phages, in some cases, resulting in very closely related phages encoding unrelated DNAPs. DNAP swapping was likely driven by selection for avoidance of host antiphage mechanisms targeting the phage DNAP that remain to be identified, and/or by selection against replicon incompatibility.