Mimivirus shows dramatic genome reduction after intraamoebal culture

Abstract

Most phagocytic protist viruses have large particles and genomes as well as many laterally acquired genes that may be associated with a sympatric intracellular life (a community-associated lifestyle with viruses, bacteria, and eukaryotes) and the presence of virophages. By subculturing Mimivirus 150 times in a germ-free amoebal host, we observed the emergence of a bald form of the virus that lacked surface fibers and replicated in a morphologically different type of viral factory. When studying a 0.40-μm filtered cloned particle, we found that its genome size shifted from 1.2 (M1) to 0.993 Mb (M4), mainly due to large deletions occurring at both ends of the genome. Some of the lost genes are encoding enzymes required for posttranslational modification of the structural viral proteins, such as glycosyltransferases and ankyrin repeat proteins. Proteomic analysis allowed identification of three proteins, probably required for the assembly of virus fibers. The genes for two of these were found to be deleted from the M4 virus genome. The proteins associated with fibers are highly antigenic and can be recognized by mouse and human antimimivirus antibodies. In addition, the bald strain (M4) was not able to propagate the sputnik virophage. Overall, the Mimivirus transition from a sympatric to an allopatric lifestyle was associated with a stepwise genome reduction and the production of a predominantly bald virophage resistant strain. The new axenic ecosystem allowed the allopatric Mimivirus to lose unnecessary genes that might be involved in the control of competitors.

Efficiency in Complexity: Composition and Dynamic Nature of Mimivirus Replication Factories

“…Our proteomic analyses indicate that the majority of proteins found in the viral factories of mimivirus are encoded by genes located in the center of the mimivirus chromosome. This observation is consistent with previous findings according to which mimivirus virions propagated under axenic conditions undergo substantial reduction of their genome that specifically occurs at the extremities of the genome and yet is not detrimental to mimivirus infectivity (35). CDSs (coding DNA sequences) are represented as blue arrowheads.…”

“…Notably, 23 proteins, including R135, a putative glucose-methanol-choline (GMC)-type oxidoreductase shown to be a component of mimivirus fibrils (35), were detected in virion particles but not in our MS studies of the relatively late 7-h VFs ( Fig. 4C; see also Tables S1 and S2 in the supplemental material).…”

“…Specifically, 201, 255, and 287 proteins were present in viral factories purified at 4, 5.5, and 7 h p.i., respectively (Fig. 4A) (35) at both 5.5 and 7 h p.i. is consistent with previous observations according to which extensive assembly processes of viral components such as membranes and capsid generation take place at these p.i.…”

“…1C), implying the presence of R135 in these factories. A possible interpretation is that R135, which was shown to be glycosylated (35), may undergo glycosylation on host membranes outside the VFs and may only then be supplemented to the assembling virions. Another interpretation could be that some proteins are present in the VFs in small amounts and are therefore undetectable.…”

Distant Mimivirus relative with a larger genome highlights the fundamental features of Megaviridae

“…This observation suggests that Poxviruses and Megaviridae, despite their considerable differences, might share a genome replication strategy (e.g., coupling replication with recombination) (20,21) that favor the rearrangement, gain, or loss of genes at the extremities of viral chromosomes. Again, the dramatic 190-kb genome reduction exhibited by a recently described spontaneous Mimivirus mutant is mainly due to large deletions occurring at both ends of the genome (22).…”