Variations on a Theme: The Diversity of Nucleus-Forming Bacteriophages

Nucleus-forming bacteriophages are a biologically unique and functionally fascinating clade of viruses. These recently characterized phages orchestrate massive and complex changes in their host bacteria by remodeling the cell wall, constructing a nucleus-like replication compartment, and using tubulin-based filaments to organize and facilitate the efficiency of the infection. Nucleus-forming phages are able to accomplish this level of organization in part due to the large sizes of their genomes, which are mostly comprised of genes of unknown function. Here I characterize divergent nucleus-forming phages in order to understand the mechanisms they share. From a phylogenetic perspective, I show that all phages that encode the major nuclear shell protein form one monophyletic clade: the chimalliviruses. Coupled with cell biology studies to characterize the replication cycles of divergent nucleus-forming phages in vivo, I propose that all chimalliviruses share the nucleus-based phage replication strategy, but that subgroups within this family each have their own variations on this theme.