Descoberta e caracterização de elementos genéticos móveis associados a um vírus gigante

Abstract

Mimiviruses are giant viruses of amoeba that can be infected by smaller viruses denominated virophages. They also possess their own mobilome, composed by different mobile genetic elements which include provirophages and transpovirons. Transpovirons are mobile DNA sequences flanked by terminal inverted repeats and phylogenetically separated in three clades corresponding to the lineage of mimivirus to which they are associated (A, B, or C). Virophages are viruses thtat require an amoeba and a giant virus to replicate. They can integrate their sequence into their hosts genomes forming provirophages. Together, transpovirons and virophages are important agents of horizontal gene transfer and promote giant viruses’ evolution. However, studies that involve these elements are still scarce around the world. In this work, we performed a genomic and phylogenetic characterization of a new Brazilian mimivirus isolate, associated to a provirophage and a transpoviron. After genomic sequencing, assembly, and genes prediction we identified the new isolate as a lineage A mimivirus (1.2 Mb; 1,164 ORFs), called mimivirus argentum, as well as two associated sequences: a transpoviron (6,624 bp; 6 ORFs) and a virophage (18,415 bp; 20 ORFs). Further analysis indicated the presence of the full virophage genome (a total of 20 ORFs) integrated into the mimivirus argentum genome. To our knowledge this is the first evidence of a virophage full genome integrated into a mimivirus sequence. The predicted genes for the virophage code the Lavidaviridae family conserved proteins, as well as proteins possibly related to integration into host genomes. The genome of detected transpoviron presents three proteins encoded by core genes from previously described transpovirons. However, its sequence and terminal inverted repeats are significantly smaller than those described for other known transpovirons. Regarding mimivirus argentum genome, it is like those from other mimiviruses and code Nucleocytoviricota conserved proteins. As well as the transpoviron and the virophage, mimivirus argentum presents a great proportion of hypothetical proteins, which are encoded by genes that did not present homologs in databases at the time of its description (ORFans). Based on this, we compared the annotation made using BLASTp and HHpred and we observed that the use of HHpred were able to decrease the number of hypothetical proteins/ORFans that would be annotated to the three studied genomes. By using HHpred, we also predicted functions and/or structures to proteins annotated as hypothetical from other transpovirons already described. The known transpovirons encode a set of 10 different proteins, with clade B presenting a greater diversity of proteins. The different comparative analysis between known transpovirons genomes, as well as phylogeny, suggest that clade A transpovirons are most conserved than the three other proposed clades and that the relationship between transpovirons and mimivirus may have been stablished before the divergence of mimiviruses lineages. Altogether our results expand the basic knowledge about elements poorly studied. Future studies, especially those of biological nature, may contribute to a better understanding of the relationship between giant viruses, their mobilome and evolution.