The Polinton-like Supergroup of Viruses: Evolution, Molecular Biology, and Taxonomy

SUMMARYPolintons are 15-20 kb-long self-synthesizing transposons that are widespread in eukaryotic, and in particular protist, genomes. Apart from a transposase and a protein-primed DNA polymerase, polintons encode homologs of major and minor jelly-roll capsid proteins, DNA-packaging ATPases, and proteases involved in capsid maturation of diverse eukaryotic viruses of kingdom . Given the conservation of these structural and morphogenetic proteins among polintons, these elements are predicted to alternate between transposon and viral lifestyles and, although virions have thus far not been detected, are classified as viruses (class ) in the phylum . Related to polintoviricetes are vertebrate adenovirids; unclassified polinton-like viruses (PLVs) identified in various environments or integrated into diverse protist genomes; virophages (), which are part of tripartite hyperparasitic systems including protist hosts and giant viruses; and capsid-less derivatives, such as cytoplasmic linear DNA plasmids of fungi and transpovirons. Phylogenomic analysis indicates that the polinton-like supergroup of viruses bridges bacterial tectivirids (preplasmiviricot class ) to the phylum that includes large and giant eukaryotic DNA viruses. Comparative structural analysis of proteins encoded by polinton-like viruses led to the discovery of previously undetected functional domains, such as terminal proteins and distinct proteases implicated in DNA polymerase processing, and clarified the evolutionary relationships within . Here, we leverage these insights into the evolution of the polinton-like supergroup to develop an amended megataxonomy that groups , PLVs (new class ''), and virophages (renamed class '') together with (new class '') in a preplasmiviricot subphylum '' sister to a subphylum including ('').