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New Mobile Genetic Element in Giant Viruses: Transpovirons

Previously, this group identified a new virophage, Sputnik 2, in the contact lens fluid of a patient with keratitis. This paper however, found that this Sputnik 2 could integrate into its viral host's genome (Lentille virus). Upon massive SOLiD genome sequencing investigating its integration, they then found a 7.4 kb linear plasmid that contained 6 ORFs. Due to the fact that a plasmid has never been found within a virus, the authors termed this mobile genetic element, a transpoviron. Although, in the prokaryotic world an analogous system exists where the P4 "phagemid" replicates like a plasmid until another phage P2 is present which then provides the capsids for P4 encapsidation, and thus viral dissemination of P4.
This element was found to have complimentary ends indicating that it exists as a circular element. Interestingly, they also found a significant number of transpoviron sequence reads that overlapped with the genomes of both the Lentille virus and Sputnik 2, evidence (that was also seen in hybridization experiments) that this element integrates into both genomes. Using both Southern blotting and in situ hybridization, they found that this element was abundant in the amoeba's cytoplasm as well. Further illustrating the importance of this element, the authors then searched the sequence databases for this element where they found these transpovirons in all known groups of Mimiviruses.
Together these results add another fantastic example to the mobilome, ie, all known mobile genetic elements. Plasmids, transposons, self-splicing elements like inteins and introns, viruses, and now, transpovirons. Given the fact that giant viruses have genes from all over the tree of life, more studies on transpovirons and other possible elements may provide perplexing views into the diversity of the microbial world.

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