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The number of novel giant viruses identified and characterized from the recently proposed order Megavirales has increased in recent years and new questions have been raised regarding viral diversity and evolution. Here, we describe the isolation and characterization of Saudi moumouvirus (SDMV), a new giant virus belonging to Mimivirus lineage B, isolated from a sewage sample taken from the King Abdulaziz University hospital in Jeddah, Saudi Arabia. SDMV presented 500 nm icosahedral particles with a 1,046,087 bp genome, which is larger than moumouvirus-like genomes which have been described in the past. The SDMV genome was predicted to encode 868 ORFs, ranging in size from 54 to 2,914 amino acids, with a mean size of 349 aa. Furthermore, this genome was predicted to encode 40 new genes (ORFans) without similarity with other sequences (ORFan L850 transcript was detected by qPCR in infected amoeba), in addition to 42 hypothetical proteins (pseudo-ORFs) with less than 100 aa, which matched other sequences in the NCBI nr database. Phylogenetic analysis showed that SDMV clustered together with mimiviruses from lineage B, including moumouvirus-like strains. It is, therefore, the third Mimivirus to be isolated in Asia and the first of group B. [ABSTRACT FROM AUTHOR]

Since the recent discovery of Samba virus, the first representative of the family Mimiviridae from Brazil, prospecting for mimiviruses has been conducted in different environmental conditions in Brazil. Recently, we isolated using Acanthamoeba sp. three new mimiviruses, all of lineage A of amoebal mimiviruses: Kroon virus from urban lake water;Amazonia virus from the Brazilian Amazon river; and Oyster virus from farmed oysters. The aims of this work were to sequence and analyze the genome of these new Brazilian mimiviruses (mimi-BR) and update the analysis of the Samba virus genome. The genomes of Samba virus, Amazonia virus and Oyster virus were 97%-99% similar, whereas Kroon virus had a low similarity (90%-91%) with other mimi-BR. A total of 3877 proteins encoded by mimi-BR were grouped into 974 orthologous clusters. In addition, we identified three new ORFans in the Kroon virus genome. Additional work is needed to expand our knowledge of the diversity of mimiviruses from Brazil, including if and why among amoebal mimiviruses those of lineage A predominate in the Brazilian environment. [ABSTRACT FROM AUTHOR]

The complexity of giant virus genomes is intriguing, especially the presence of genes encoding components of the protein translation machinery such as transfer RNAs and aminoacyl-tRNA-synthetases; these features are uncommon among other viruses. Although orthologs of these genes are codified by their hosts, one can hypothesize that having these translation-related genes might represent a gain of fitness during infection. Therefore, the aim of this study was to evaluate the expression of translation-related genes by mimivirus during infection of Acanthamoeba castellanii under different nutritional conditions. In silico analysis of amino acid usage revealed remarkable differences between the mimivirus isolates and the A. castellanii host. Relative expression analysis by quantitative PCR revealed that mimivirus was able to modulate the expression of eight viral translation-related genes according to the amoebal growth condition, with a higher induction of gene expression under starvation. Some mimivirus isolates presented differences in translation-related gene expression; notably, polymorphisms in the promoter regions correlated with these differences. Two mimivirus isolates did not encode the tryptophanyl-tRNA in their genomes, which may be linked with low conservation pressure based on amino acid usage analysis. Taken together, our data suggest that mimivirus can modulate the expression of translation-related genes in response to nutrient availability in the host cell, allowing the mimivirus to adapt to different hosts growing under different nutritional conditions. [ABSTRACT FROM AUTHOR]

The history of giant viruses began in 2003 with the identification of Acanthamoeba polyphaga mimivirus. Since then, giant viruses of amoeba enlightened an unknown part of the viral world, and every discovery and characterization of a new giant virus modifies our perception of the virosphere. This notably includes their exceptional virion sizes from 200 nm to 2 µm and their genomic complexity with length, number of genes, and functions such as translational components never seen before. Even more surprising, Mimivirus possesses a unique mobilome composed of virophages, transpovirons, and a defense system against virophages named Mimivirus virophage resistance element (MIMIVIRE). From the discovery and isolation of new giant viruses to their possible roles in humans, this review shows the active contribution of the University Hospital Institute (IHU) Mediterranee Infection to the growing knowledge of the giant viruses' field. [ABSTRACT FROM AUTHOR]