Content area
Full Text
REVIEWS
The discovery of the Archaea was a significant breakthrough in the recent history of biology. Whereas cell-ultrastructure studies had initially suggested a division of living organisms into eukaryotes and prokaryotes, molecular sequence analyses pioneered by Carl Woese in the 1970s revealed the existence of three different classes of ribosomal RNAs and ribosomes in cellular organisms. This discovery led to the replacement of the prokaryote/eukaryote dichotomy by a trinity of domains, the Archaea, Bacteria and Eukarya1. Subsequently, ribosomal RNA sequence comparisons led to the division of the archaeal domain into two main kingdoms, the Crenarchaeota and the Euryarchaeota2.
Over the past three decades we have accrued a broad knowledge of the biological diversity of the Archaea. This includes an outline of their physiology, biochemistry and molecular biology, and many insights into their evolutionary relationships with the Bacteria and the Eukarya3. Although Archaea resemble Bacteria in their cellular ultrastructure and genome organization, their DNA replication, transcription and translation machineries show many similarities to their eukaryotic counterparts. In addition, other features seem to have either arisen, or have been exclusively conserved, within the archaeal domain (for example, ether-linked membranes). In this context, archaeal viruses are particularly interesting.
The first two archaeal viruses that were isolated visually resembled bacteriophage T4 and other members of the family Myoviridae, with icosahedral heads, contractile helical tails and linear, double-stranded (ds) DNA genomes4,5. Subsequently, a few head-tail archaeal viruses were reported with non-contractile tails similar to lambdoid bacteriophages of the family Siphoviridae6,7.
Based on these initial studies, it was inferred, albeit erroneously, that archaeal viruses constituted a variety of the ubiquitous head-tail bacteriophages.
Recently however, this view has changed radically. Electron-microscopy investigations of samples collected
from natural environments that contain predominantly archaea, and the enrichment cultures derived therefrom, revealed that the head-tail phenotype is rare among the archaeal viruses (for reviews, see REFS 8,9). In fact, cultured archaeal viruses, which to date all have dsDNA genomes, exhibit a range of virion morphotypes, most of which have not been observed before for any dsDNA virus. There are exceptional forms, including fusiforms, droplet and bottle shapes, and linear and spherical virions, with more complex virions combining features of these different forms. Moreover, genome-sequence analyses have demonstrated that most of the archaeal viruses are...