UNUSUAL LIFE STYLE OF GIANT CHLORELLA VIRUSES

Paramecium bursaria chlorella virus (PBCV-1) is the prototype of a family of large, icosahedral, plaque-forming, dsDNA viruses that replicate in certain unicellular, eukaryotic chlorella-like green algae. Its 330-kb genome contains ∼373 protein-encoding genes and 11 tRNA genes. The predicted gene products of ∼50% of these genes resemble proteins of known function, including many that are unexpected for a virus, e.g., ornithine decarboxylase, hyaluronan synthase, GDP-D-mannose 4,6 dehydratase, and a potassium ion channel protein. In addition to their large genome size, the chlorella viruses have other features that distinguish them from most viruses. These features include: ( a ) The viruses encode multiple DNA methyltransferases and DNA site-specific endonucleases. ( b ) The viruses encode at least some, if not all, of the enzymes required to glycosylate their proteins. ( c ) PBCV-1 has at least three types of introns, a self-splicing intron in a transcription factor-like gene, a spliceosomal processed intron in its DNA polymerase gene, and a small intron in one of its tRNA genes. ( d ) Many chlorella virus-encoded proteins are either the smallest or among the smallest proteins of their class. ( e ) Accumulating evidence indicates that the chlorella viruses have a very long evolutionary history.