James L. Van Etten,
Department of Plant Pathology, University of Nebraska, Lincoln, NE USA
Many large (150 - 190 nm in diameter), polyhedral, plaque-forming viruses that replicate in certain isolates of unicellular, eukaryotic, chlorella-like green algae have been isolated and partially characterized in the past 15 years. The viral particles contain at least 50 structural proteins and a lipid component located inside the outer glycoprotein capsid. The 330,740 base pair genome of the prototype virus PBCV-1 has been sequenced and is predicted to encode 377 protein encoding genes and ten tRNA genes. Many of these viral encoded genes were unexpected and have never been found in a virus genome. In addition to their large genome size, the chlorella viruses have at least three other unusual properties. i) The viruses encode multiple DNA methyltransferases and DNA site-specific (restriction) endonucleases. ii) PBCV-1 virions, like many viruses, contain glycoproteins. However, unlike other viruses that contain glycoproteins, PBCV-1 encodes at least part, if not its entire, glycosylation machinery. iii) PBCV-1 has two different types of introns; a self-splicing intron in a transcription factor-like gene and a splicesomal processed type of intron in its DNA polymerase gene. In addition, one of the tRNA genes is predicted to contain a small intron.