Conserved sequence in Mimivirus gene promoters
In an attempt to further elucidate the evolutionary origin of Mimivirus using the viral genome sequence, Suhre et al analyzed the 5' upstream regions of all Mimivirus genes in search of transcriptional motifs. Surprisingly, 403 of the 911 (45%) predicted Mimivirus genes exhibited a perfectly conserved AAAATTGA motif within their 150-nt upstream region. The motif tends to be present in the promoters of genes that are transcribed from the predicted leading strand and that are involved in transcription and protein translation. Careful comparative analyses of these promoter regions with those of unicellular eukaryotes suggest that the motif is equivalent to the TATA box in eukaryotes. The discovery of this promoter region suggests that it is derived from an ancestral promoter sequence that may predate the evolutionary divergence of the eukaryotic kingdoms.
Suhre K, Audic S, Claverie JM. “Mimivirus gene promoters exhibit an unprecedented conservation among all eukaryotes.” Pro Natl Acad of Sci U S A. 2005 Oct 11; 102(41): 14689-93.
Gene and genome duplication in Mimivirus
A recent sequence analysis of the Mimivirus genome revealed that at least one-third of the Mimivirus genes have at least one paralogue in the genome, highlighting the major role played by both large segmental and smaller gene duplication events in shaping the Mimivirus genome. Identification of these paralogues allowed investigator Karsten Suhre to suggest functions for previously orphan genes. Many of these genes were identified by the author as playing significant roles in host-virus interactions. Specifically, these genes are likely to interfere with host cellular processes such as protein degradation, transcriptional control, and cell regulatory processes.
Suhre K. “Gene and genome duplication in Acanthamoeba polyphaga Mimivirus.” J Virol. 2005 Nov; 79(22): 14095-101.
In order to better understand the evolutionary history of Mimivirus, efforts are being made to identify related viruses and potential new members of the Mimiviridae family. A similarity sequence search of all publicly available sequences revealed that Mimivirus is most closely related to large DNA viruses present in the Sargasso Sea . Given that these viruses were collected for sequencing by environmental sampling, they are likely ubiquitous in these marine waters. Isolation and further analysis of these viruses may prove invaluable in uncovering the still unclear origin of Mimivirus.
Ghedin E, Claverie JM. “Mimivirus relatives in the Sargasso sea .” Virol J. 2005 Aug 16; 2: 62-7.
A viral intein in Mimivirus
One interesting observation of Mimivirus has been the identification of a genomic sequence characteristic of an intein in the DNA polymerase PolB gene. An intein is a “protein intron,” a protein equivalent of a transcript intron. An intein removes itself from the host protein through an autocatalytic protein-splicing. The Mimivirus intein displays important characteristic sequential motifs of archaeal inteins, but the PolB sequences are most similar to eukaryotes. As a result, the discovery of an intein in Mimivirus implicates that DNA viruses may have been the central reservoir of inteins throughout the course of evolution.
Ogata H, Raoult D, Claverie JM. “A new example of viral intein in Mimivirus.” Virol J. 2005 Feb 11;2(1):8.
Alternate view on Mimivirus phylogenetic position
A comment to “The 1.2-megabase genome sequence of Mimivirus” was recently published to provide an alternate opinion on the evolutionary influence of Mimivirus. The commentary argues that Mimivirus acquired its diversity of genes not as an early evolutionary species but as a more recently emerged gene parasite. It offers horizontal gene transfer as a more probably mechanism of gene acquisition by Mimivirus and places the species under the eukaryotic genus of Entamoeba . The response suggests that future research should concentrate on the abilities of Mimivirus to acquire an assortment of genes and increase its genome size.
Moreira D, Lopez-Garcia P. “Comment on ‘The 1.2-megabase genome sequence of Mimivirus.” Science. 2005 May 20;308(5725):1114.