Elsevier

Virus Research

Volume 117, Issue 1, April 2006, Pages 145-155
Virus Research

Review
Genomic and evolutionary aspects of Mimivirus

https://doi.org/10.1016/j.virusres.2005.07.011 Get rights and content

Abstract

We recently described a giant double stranded DNA virus called Mimivirus, isolated from amoebae, which might represent a new pneumonia-associated human pathogen. Its unique morphological and genomic characteristics allowed us to propose Mimivirus as a member of a new distinct Nucleocytoplasmic Large DNA viruses family, the Mimiviridae. Mimivirus-specific features, namely its size and its genomic complexity, ranged it between viruses and cellular organisms. This paper reviews our current knowledge on Mimivirus structure, life cycle and genome analysis and discusses its putative evolutionary origin in the tree of species of the three domains of life.

Section snippets

Discovery

Cultures of amoebae with water from a cooling tower during an outbreak of pneumonia in Bradford, England, led T.J. Rowbotham to isolate a series of Legionella-like Amoebal Pathogens (LLAP), among which one was Gram-positive and was first called “Bradford coccus” (unpublished results). Different attempts in our laboratory to identify this amoebae-associated bacterium by universal bacterial 16S ribosomal RNA (rRNA) PCR amplification and sequencing reproducibly failed. Initial electron microscopy

Morphological characteristics of the viral particle

The main characteristics of Mimivirus particles observed by transmission electron microscopy are its size of about 650 nm, its icosahedral capsid and the presence of fibrils surrounding the capsid. From outside to inside, the capsid seems to be composed of several membrane layers inside which condensed core material could be observed (Figs. 1B and 2A). In some cases, the core material appeared uncondensed (Fig. 2B). Other morphological aspects of Mimivirus particles are shown in Fig. 2C and D.

General features

The genome of Mimivirus is a linear dsDNA of 1.2 Mb long which was entirely sequenced (GenBank accession number AY653733 ). The viral genome might adopt a circular topology upon annealing of the 900 nt inverted repeats found at each end (Raoult et al., 2004). This genome is larger than the genome of several bacteria and archeon parasites (Koonin, 2005) and is the largest of the dsDNA viral genomes described (Table 2). Apart from this exceptional large size, the Mimivirus genome is mainly

Is Mimivirus a cellular organism or a virus?

By its size of about 650 nm, Mimivirus is not an ultrafiltrable agent as commonly defined for most viruses, but is rather comparable to the size of small intracellular bacteria such as Rickettsia conorii or Tropheryma whipplei (Raoult, 2005). The 1.2 Mb size of its genome is comparable to the genomes of these bacteria, 1.27 Mb for R. conorii (Ogata et al., 2001), and 0.9 Mb for T. whipplei (Bentley et al., 2003). Strand asymmetry for gene transcription was present in Mimivirus as in bacteria, which

Acknowledgements

Lina Barassi, Bernard Campagna, Christophe Fraisier, Amélie Honstettre, and Catherine Robert are gratefully acknowledge for their expert technical assistance.

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