1. The Mimivirus Giant double stranded DNA virus Discovered in amoebas
Belongs to entirely new family of viruses
In March of 2003, a group of French researchers reported finding the world’s largest known virus inside an amoeba. This was a serendipitous discovery that had been part of an effort to track down the source of a pneumonia outbreak in England. An examination of samples taken from a water tower in one of the affected communities showed bacteria-sized particles inside amoebas that were Gram-positive, indicating the presence of a particular type of bacteria. However, when investigators used PCR to amplify a ribosomal RNA gene universally found in bacteria, they failed, which argued strongly against the presence of bacteria. Upon closer inspection , the particle inside the amoeba was identified as the world’s largest known virus. At 400 nm in diameter, it is comparable in size to some bacteria. Researchers dubbed this virus Mimivirus for “Mimicking microbe.” Based on comparisons with other viral protein sequences, Mimivirus was placed in a new viral family that is most closely related to other large double-stranded DNA viruses that infect eukaryotic cells.
From Figure in Bruich, S. (2004) “Giant virus sequenced” in (ScienceNow October 14, 2004).
© 2005 Prentice Hall Inc. / A Pearson Education Company / Upper Saddle River, New Jersey 07458

2. Mimivirus genome 1.18 Mb 1262 putative ORFs
Larger than some bacterial genomes
1262 putative ORFs
More genes than some bacterial genomes
50 genes not found in any other viruses
The genome of Mimivirus was fully sequenced in At 1.2 million base pairs, the Mimivirus genome is more than double the size of some bacterial genomes. Fifty of the genome’s 1,260 putative genes have never previously been observed in viruses. Other unusual features of the virus included genes for DNA repair enzymes and components for every aspect of translation except ribosomal subunits. The function of various Mimivirus genes based on their COG membership is shown in the slide. Letters in the legend represent the following: E for amino acid metabolism and transport, F for nucleotide metabolism and transport, J for translation, K for transcription, L for replication (including recombination and repair), M for cell wall and membrane biogenesis, N for cell motility, O for posttranslational modifications, chaperones, and protein turnover, Q for biosynthesis of secondary metabolites, transport, and catabolism, R for general function prediction, and S for unknown function.
From Figure 1 in Raoult, D. et al. (2004) “The 1.2-Megabase Genome Sequence of Mimivirus” Sciencexpress/ October 2004/Page 1/ /science
© 2005 Prentice Hall Inc. / A Pearson Education Company / Upper Saddle River, New Jersey 07458

3. A controversial hypothesis
Did giant viruses play a role in the evolution of the nucleus?
Nucleus-virus similarities
Linear chromosomes
No ribosomes
Membrane surrounds DNA replication
The concatenated sequences of seven universally conserved genes were used to build a phylogenetic tree that would position the Mimivirus in the context of the Universal Tree of Life. The results of this analysis are shown in the slide. The phylogenetic tree suggests that the Mimivirus and eukaryotes shared a common ancestor that pre-dates the emergence of Eukarya. This is consistent with a controversial hypothesis that postulates the emergence of the nucleus from a large DNA virus. Proponents of this hypothesis argue that linear chromosomes, telomeres, the absence of ribosomes, and membrane-enclosed DNA replication are all features shared by eukaryotic nuclei and many double-stranded DNA viruses. While many biologists continue to be skeptical of this hypothesis, the analysis of the Mimivirus genome certainly raises some intriguing possibilities.
From Figure 2 in Raoult, D. et al. (2004) “The 1.2-Megabase Genome Sequence of Mimivirus” Sciencexpress/ October 2004/Page 1/ /science
© 2005 Prentice Hall Inc. / A Pearson Education Company / Upper Saddle River, New Jersey 07458