Reconsider the Mimivirus... Its greater than we thought

A review of: Legendre et al. (2011). Breaking the 1000-gene barrier for Mimivirus using ultra-deep genome and transcriptome sequencing. Virology Journal 8:99.

The Mimivirus is a giant dsDNA virus which infects Acanthamoeba species. It is one of the largest and most complex viruses known, and is the latest addition to the family of nucleocytoplasmic large DNA viruses. Its 1.2-Mb genome was initially predicted to encode 917 genes. However, subsequent analysis from this report and other sources has increased this number to 1018, which is 11% greater than initial predictions. Having such a large genome allows the virus to perform most of the functions of a living cell. This expanding knowledge of this virus and its great complexity has raised many questions about the division between viruses and single-celled organisms, and of course brings their evolution into question.

This short report explains how the Mimivirus has been analysed in greater depth than ever before using SOLIDTM technology. The study explores the complexity of the virus by a series of new and developed techniques and in depth analyses. RNA-Sequencing of the Mimivirus transcriptome was performed during the infectious cycle and a complete genome re-sequencing was carried out. Transcriptome analysis allowed for the correction of some phase-shifting sequencing errors, which in turn, presented new genes. These genes had been previously lost where these errors caused some ORFs (open reading frames) to be merged where a stop codon should have been seen to separate them. Limitations were also overcome by performing re-sequencing and re-annotation of the genome which created lots of new data. This clearly would improve the quality of the Mimivirus genome sequence, but it also allowed for identification of polymorphic genomic positions and resulted in the discovery of previously overlooked genes. At the end of the study, the gene count of the Mimivirus had been increased to 1018 – an incredible 11% greater than previously thought.

This report allows for further study on giant viruses to be completed efficiently alongside the questions it has raised concerning the division between viruses and single-celled organisms. This work is very important progress in developing our knowledge of viruses and their evolution. It is also important in improving the techniques we use to study them so that we can research them thoroughly and correctly. The Mimivirus is as large as several bacteria, has a genome bigger than a number of bacteria and it contains genetic information that was previously unknown in viruses. Some of this information, such as genes coding for nucleotide and amino acid synthesis, is even lacking in some bacteria. The role and evolution of viruses must be reconsidered as they quickly develop and so too does our knowledge of them...


Relevant further reading..
- La Scola, B. et al. (2003) A giant virus in amoebae. Science 299:2033.

- Suzan-Monti M. (2005) Genomic and evolutionary aspects of Mimivirus. Virus Res.

- Claverie JM. et al. (2006) Mimivirus and the emerging concept of giant virus. Virus Red. 117: 113-144.