GENOME
Much of what is known about mimivirus is based on analyses of the viral genome. In 2004, Raoult et al published the complete 1.2-Megabase genome sequence of the virus, confirming its status as a true giant among viruses and demonstrating for the first time that the mimivirus genome is larger that the genomes of several bacteria and archeon parasites. In addition to pure size, the viral genome is unique for its large coding capacity of 90.5%, with an estimated 1262 genes based on the number of putative open reading frames. Interestingly, careful computational analysis by Raoult et al revealed that only 298 of the 1262 (24%) genes could be assigned homology-based functions, a notable difference from the typical results of newly sequenced prokaryotic genomes, which tend to assign homology-based function to about 70% of predicted genes. Thus, while it is clear that mimivirus blurs the distinction between viruses and cells, there is much to be learned about the mimivirus genome. Still, comparative analyses of the known genes highlight the virus's homology to the nucleocytoplasmic large DNA viruses (NCLDVs) and to other prokaryotes and eukaryotes.
Previous comparative analyses of the other NCLDV families, which include Poxviridae, Phycodnaviridae, Asfarviridae, and Iridoviridae, have identified 31 core genes present in all or most members. Subdivided into four classes, from most to least evolutionarily conserved, these core genes are involved in viral replication and transcription as well as virion biogenesis. Of the 31 genes, the mimivirus genome possesses 26, indicating a clear relationship to these other viruses. At the same time, the absence of five core genes and presence of such a large number of unique genes within the genome underscore mimivirus's classification in a distinct viral family.
The homology of other mimivirus genes to many prokaryotic and eukaryotic genes is also an important trademark of mimivirus. Among the viral genes with eukaryotic homologs are those involved in tRNA modification, translation, molecular chaperones and protein folding, amino acid metabolism, protein modification, and lipid metabolism. By contrast, genes predicted to deal with nucleotide synthesis and polysaccharide metabolism tend to be closer to bacterial homologs. Lastly, genes encoding DNA repair enzymes show homology to both eukaryotic and prokaryotic genes. Taken independently, the presence of these genes in a viral genome is well established, since other viral genomes encode some of them as well; however, no other virus possesses the large and diverse combination of these genes that mimivirus does. Although the function of all these genes in the context of the mimivirus genome remains unclear, it is likely that their expression is related to an infection strategy that profoundly alters cellular functions.
Since the initial genomic analysis presented by Raoult et al, further investigation of several mimivirus genomic features has been reported. Several expanded gene families sharing conserved motifs have been identified and the role of gene duplication in mimivirus evolution has been explored more fully. As pieces of the genomic puzzle are uncovered, greater insight into viral processes, viral evolution, and the relevance of mimivirus to human disease will undoubtedly be revealed.
Picture: Raoult et al (2004)
Created: 11/28/05
Last Modified: 11/28/05
Creators: David Berg and Kim Tran
Humans and Viruses, Autumn 2005
