Mimivirus and the emerging concept of "giant" virus

TL;DR

The discovery of Mimivirus, the largest known DNA virus, challenges traditional distinctions between viruses and cellular organisms, prompting a reevaluation of microbial classification and the proposal of a new category called 'girus'.

Contribution

This paper introduces the concept of 'girus' to classify giant viruses with complex genomes, bridging the gap between viruses and cellular life forms.

Findings

Mimivirus has a particle size over 400 nm and a genome of 1.2 million bp.

It contains genes typically associated with cellular organisms, such as aminoacyl-tRNA synthetases.

The discovery suggests a need to revise microbial taxonomy and the definition of life forms.

Abstract

The recently discovered Acanthamoeba polyphaga Mimivirus is the largest known DNA virus. Its particle size (>400 nm), genome length (1.2 million bp) and large gene repertoire (911 protein coding genes) blur the established boundaries between viruses and parasitic cellular organisms. In addition, the analysis of its genome sequence identified new types of genes not expected to be seen in a virus, such as aminoacyl-tRNA synthetases and other central components of the translation machinery. In this article, we examine how the finding of a giant virus for the first time overlapping with the world of cellular organisms in terms of size and genome complexity might durably influence the way we look at microbial biodiversity, and force us to fundamentally revise our classification of life forms. We propose to introduce the word "girus" to recognize the intermediate status of these giant DNA viruses, the genome complexity of which make them closer to small parasitic prokaryotes than to regular viruses.