Abstract
Pandoraviruses are giant viruses of ameba with 1 μm-long virions. They have an ovoid morphology and are surrounded by a tegument-like structure lacking any capsid protein nor any gene encoding a capsid protein. In this work, we studied the ultrastructure of the tegument surrounding Pandoravirus massiliensis virions and noticed that this tegument is composed of a peripheral sugar layer, an electron-dense membrane, and a thick electron-dense layer consisting in several tubules arranged in a helicoidal structure resembling that of cellulose. Pandoravirus massiliensis particles were stained by Calcofluor white, a fluorescent dye of cellulose, and the enzymatic treatment of particles by cellulase showed the degradation of the viral tegument. We first hypothesized that the cellulose tegument could be synthesized by enzymes encoded by the virus. Bioinformatic analyses revealed in P. massiliensis, a candidate gene encoding a putative cellulose synthase, with a homology with the BcsA domain, one of the catalytic subunits of the bacterial cellulose synthase, but with a low level of homology. This gene was transcribed during the replicative cycle of P. massiliensis, but several arguments run counter to this hypothesis. Indeed, even if this gene is present in other pandoraviruses, the one of the strain studied is the only one to have this BcsA domain and no other enzymes involved in the synthesis of cellulose could be detected, although we cannot rule out that such genes could have been undetected among the large proportion of Orfans of pandoraviruses. As an alternative, we investigated whether P. massiliensis could divert the cellulose synthesis machinery of the ameba to its own account. Indeed, contrary to what is observed in the case of infections with other giant viruses such as mimiviruses, it appears that the transcription of the ameba, at least for the cellulose synthase gene, continues throughout the growth phase of particles of P. massiliensis. Finally, we believe that this scenario is more plausible. If confirmed, it could be a unique mechanism in the virosphere.
Highlights
Giant viruses of ameba are phylogenetically closely relative to the Nucleo – Cytoplasmic Large DNA Viruses (NCLDVs), within a new proposed order named Megavirales (Colson et al, 2013)
Other recent prospecting studies reported the isolation of seven additional strains: Pandoravirus quercus, isolated from a soil sample collected in Marseille (France); Pandoravirus neocaledonia, isolated from the brackish water of a mangrove near Noumea airport (New Caledonia), Pandoravirus macleodensis, isolated from a freshwater pond near Melbourne (Australia) (Legendre et al, 2018), Pandoravirus celtis, isolated from a soil sample collected in Marseille (Legendre et al, 2019) and three new pandoraviruses isolated from water samples in Brazil (Pereira Andrade et al, 2019)
Several markers of sugars, the degradation of virions by cellulase and the use of appropriate negative controls clearly confirmed the cellulosic nature of the Pandoravirus viral tegument
Summary
Giant viruses of ameba are phylogenetically closely relative to the Nucleo – Cytoplasmic Large DNA Viruses (NCLDVs), within a new proposed order named Megavirales (Colson et al, 2013). Pandoraviruses harbor specific morphological and genetic features, including ovoid-shaped particles with an ostiole-like apex and measuring ∼1.0 μm in length and ∼0.5 μm in diameter, classifying them as the second largest particles after those of Pithovirus sibericum (Legendre et al, 2014). These viruses have a doublestranded DNA genome, up to 2.5 Mb (for P. salinus), the largest genome ever described to date in the virosphere (Philippe et al, 2013). In 2015–2016, we isolated three new Pandoravirus strains from sewage and soda lake water samples collected in Brazil, by coculture on A. castellanii. Other recent prospecting studies reported the isolation of seven additional strains: Pandoravirus quercus, isolated from a soil sample collected in Marseille (France); Pandoravirus neocaledonia, isolated from the brackish water of a mangrove near Noumea airport (New Caledonia), Pandoravirus macleodensis, isolated from a freshwater pond near Melbourne (Australia) (Legendre et al, 2018), Pandoravirus celtis, isolated from a soil sample collected in Marseille (Legendre et al, 2019) and three new pandoraviruses isolated from water samples in Brazil (Pereira Andrade et al, 2019)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
