Phycodnaviruses

The family Phycodnaviridae encompasses a morphologically similar, but biologically and genetically diverse, group of large icosahedral, double-stranded DNA (dsDNA) viruses that infect algae. The viruses have genomes ranging from 160 to 560 kbp. Currently, the family consists of six genera that was initially based on host range and subsequently supported by sequence comparisons. Often the genera can be distinguished by additional properties, for example, lytic versus lysogenic lifestyles or linear versus circular genomes. The phycodnavirus family is monophyletic with branches for each genus, but they have evolutionary roots with four other families of large dsDNA viruses (poxviruses, iridoviruses, asfarviruses, and mimiviruses) collectively referred to as nucleocytoplasmic large DNA viruses. The genomes of members in three genera of phycodnaviruses have been sequenced. These sequences reveal more than 1000 unique protein-encoding genes, of which ~30% match some sequences in the public databases. Surprisingly, the three sequenced viruses only have 14 genes in common. Many of the virus genes encode proteins that are completely unexpected for a virus. Often the virus-encoded proteins are either the smallest or among the smallest proteins of their class. Consequently, several virus-encoded proteins are the subject of biochemical and structural studies, including an ATP-dependent DNA ligase, a type II DNA topoisomerase, a prolyl-4-hydroxylase, and a potassium ion channel protein.