Cryo-EM structure of ssDNA bacteriophage Phi CjT23 provides insight into early virus evolution

The origin of viruses remains an open question. While lack of detectable sequence similarity hampers the analysis of distantly related viruses, structural biology investigations of conserved capsid protein structures facilitate the study of distant evolutionary relationships. Here we characterize the lipidcontaining ssDNA temperate bacteriophage Phi CjT23, which infects Flavobacterium sp. (Bacteroidetes). We report Phi CjT23-like sequences in the genome of strains belonging to several Flavobacterium species. The virion structure determined by cryogenic electronmicroscopy reveals similarities to members of the viral kingdom Bamfordvirae that currently consists solely of dsDNA viruses with a major capsid protein composed of two upright beta-sandwiches. The minimalistic structure of Phi CjT23 suggests that this phage serves as a model for the last common ancestor between ssDNA and dsDNA viruses in the Bamfordvirae. Both Phi CjT23 and the related phage FLiP infect Flavobacterium species found in several environments, suggesting that these types of viruses have a global distribution and a shared evolutionary origin. Detailed comparisons to related, more complex viruses not only expand our knowledge about this group of viruses but also provide a rare glimpse into early virus evolution.

Automated summary

This study characterizes the Phi CjT23 bacteriophage, which infects Flavobacterium sp. and has similarities to dsDNA viruses in the Bamfordvirae kingdom. The minimalistic structure of Phi CjT23 serves as a model for the last common ancestor between ssDNA and dsDNA viruses. Additionally, Phi CjT23 and the related phage FLiP have a global distribution and a shared evolutionary origin.