James L. Van Etten [1]

Russel H. Meints [2]

[1] Department of Plant Pathology University of Nebraska, Lincoln, Nebraska 68583-0722; e-mail: jvanetten@unlnotes.unl.edu; [2] Department of Botany and Plant Pathology, Center for Gene Research and Biotechnology Oregon State University, Corvallis, Oregon 97331-2906

Key Words chlorella viruses, brown algal viruses, DNA restriction/modification enzymes, virus-encoded glycosylation, lysogeny

* Abstract Paramecium bursaria chlorella virus (PBCV-1) is the prototype of a family of large, icosahedral, plaque-forming, double-stranded-DNA-containing viruses that replicate in certain unicellular, eukaryotic chlorella-like green algae. DNA sequence analysis of its 330, 742-bp genome leads to the prediction that this phycodnavirus has 376 protein-encoding genes and 10 transfer RNA genes. The predicted gene products of 40% of these genes resemble proteins of known function. The chlorella viruses have other features that distinguish them from most viruses, in addition to their large genome size. These features include the following: (a) The viruses encode multiple DNA methyltransferases and DNA site-specific endonucleases; (b) PBCV-l encodes at least part, if not the entire machinery to glycosylate its proteins; (c) PBCV-1 has at least two 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. Unlike the chlorella viruses, large double-stranded--DNA-containing viruses that infect marine, filamentous brown algae have a circular genome and a lysogenic phase in their life cycle.

INTRODUCTION

Since the early 1970s, viruses or virus-like particles have been reported in at least 44 taxa of eukaryotic algae, which include members in 10 of the 14 classes of alga [171]. However, most of these reports described isolated accounts of microscopic observations, and the virus particles were not characterized further for many reasons. This situation changed in the early 1980s with the discovery of large double-stranded-DNA (dsDNA)-containing viruses that infect and replicate in certain strains of unicellular, eukaryotic, exsymbiotic chlorella-like green algae (also referred to as zoochlorellae) and more recently with the finding of tractable virus systems in brown algae.

In 1978, Kawakami & Kawakami [60] described the appearance of large (180-nm diameter), lytic viruses in zoochlorellae after these algae were released from the protozoan Paramecium bursaria. No virus particles were detected in zoochlorellae growing symbiotically inside the paramecium. Independently, lytic viruses were described in zoochlorellae isolated from the green coelenterate Hydra viridis [93, 173] and also from P bursaria [174]. As in the 1978 report, viruses appeared only after the zoochlorellae were separated from their hosts. Fortunately, the zoochlorellae from P bursaria can be grown free of the paramecium in culture, and ensuing experiments have revealed that these cultured algae serve as hosts for many closely related viruses. These lytic chlorella viruses can be produced in large quantities and assayed by plaque formation with standard bacteriophage techniques [159, 167]. The chlorella viruses have several unique properties including the following: (a) They have large, 330- to 380-kb genomes [12 8, 195], and analyses of the recently sequenced 330,742-bp genome of the prototype chlorella virus, Paramecium bursaria chlorella virus 1 (PBCV-l), have revealed many interesting and unexpected putative genes....