Abstract
MIMIVIRE is a defense system described in lineage A Mimivirus (Mimiviridae family) that mediates resistance against Zamilon virophage. It is composed of putative helicase and nuclease associated with a gene of unknown function called R349, which contains four 15 bp repeats homologous to the virophage sequence. In a previous study, the silencing of such genes restored virophage susceptibility. Moreover, the CRISPR Cas-4 like activity of the nuclease has recently been characterized. In this study, a recently isolated Mimivirus of lineage A with R349 gene lacking 3 of 4 repeats was demonstrated to be susceptible to Zamilon. To reinforce the importance of the R349 gene in the MIMIVIRE system, we developed and presented, for the first time to our knowledge, a protocol for Mimivirus genomic editing. By knocking out R349 gene in a Mimivirus lineage A, we observed the replication of Zamilon, indicating that this gene is critical in the resistance against this specific group of virophages.
Highlights
In recent years, several studies have shown that the endogenization of pathogen sequences to prevent their multiplication is a main mechanism of defense for living organisms
We found that this 21-nucleotide-long stretch presents 3 mismatches compared to the original 28 nucleotide sequence located in open reading frame 4 (ORF 4) of the Zamilon genome and the R349 of the other mimiviruses from the group A (Figure 2 and Supplementary Figure S2)
The repetitive motifs are lost in the Mimivirus U306 MIMIVIRE copy as the ORF 364 gene only contains a single copy of the 15-nucleotide sequence that was identical to Zamilon, versus four repeated copies described in the MIMIVIRE of the other mimiviruses lineage A (Figure 2)
Summary
Several studies have shown that the endogenization of pathogen sequences to prevent their multiplication is a main mechanism of defense for living organisms This phenomenon has been intensively discussed in the context of vertebrates evolution, where the number of integrated retroviruses reaches several thousand per organism (Colson et al, 2014). Bacteria and archaea have evolved several immune strategies to defend themselves against viruses, which are the most abundant biological entities in the biosphere (Suttle, 2005, 2007; Rohwer and Thurber, 2009; Horvath and Barrangou, 2010) These immune strategies have been classified into innate and adaptive defense systems (Koonin et al, 2016). The innate immunity systems are represented by diverse types of restriction modification (RM) systems, while the CRISPR (clustered regularly interspaced short palindromic repeats)-Cas (CRISPR-associated genes) system is the unique adaptive immune mechanism that mediates defense against viruses (Orlowski and Bujnicki, 2008; Karginov and Hannon, 2010)
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