Thursday, March 5, 2015
The enigmatic biology of multipartite viruses
INRA, UMR BGPI, CIRAD TA-A54/K, Campus International de Baillarguet / CNRS, UMR MIVEGEC, IRD, 911 Avenue Agropolis, B.P. 64501, 34394 Montpellier
Multipartite viruses have their genome composed of two or more nucleic acid segments, each encapsidated individually. A classical view in virology, not to say a dogma, assumes that the viral replication cycle occurs within individual cells, where the whole viral genome information is replicated, and is then reiterated in successively infected cells during host invasion. In the context of multipartite viruses, this view implies that at least one copy of each of the genome segments must enter in each of the infected cells, and thus that a large number of virus particles must enter together. The genome of the Faba bean necrotic stunt virus (FBNSV, Family Nanoviridae) is composed of 8 ssDNA circles of about 1000 bases, each encapsidated in an individual virus particle. We have previously shown that each of the eight segments reproducibly accumulates at a specific relative frequency in infected host plants, some representing around 30 % of the total viral DNA within an infected plant and others not exceeding 2%. In this situation, it is difficult to conceive how FBNSV can actually transmit the whole genome information both from cell to cell and from host to host. Because all segments have a similar size and are encapsidated by the same coat protein, it is likely that all viral particles are alike, whatever segment they contain. If so, the segments enter cells indifferently, and their frequency of cell-entry is solely dependent on their frequency within the population. In such a situation, we could calculate that a successful infection of 95% of the susceptible cells would require the entry of nearly 200 particles per cell. This figure illustrates the enormous cost that FBNSV might bear at each cell-to-cell transmission step. Alternatively, a view challenging the above mentioned dogma is that this virus might infect individual cells with subgroups of genome segments, partial genome information being replicated at distinct location within a host. This may alleviate the cost at cell-to-cell passage but would imply a sort of unknown viral communication or complementation in between these subgroups of segments to maintain the integrity and coordination of the genome information. In any cases, the actual functioning of FBNSV is an enigma (as is that of other multipartite viruses), because it is hard to conceive that a virus could force hundreds of particles in each newly colonized cells, or that the genome could function with separate subunits in distinct cells. We are currently developing tools to test the above alternatives.
Contact : Alexandre Martinière
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