Project description:Giant viruses are extraordinary members of the virosphere due to their structural complexity and high diversity in gene content. Haptophytes are ecologically important primary producers in the ocean, and all known viruses that infect haptophytes are giant viruses. Our in-depth electron microscopic, phylogenomic and virion proteomic analyses of two haptophyte-infecting giant viruses, Haptolina ericina virus RF02 (HeV RF02) and Prymnesium kappa virus RF02 (PkV RF02), unravel their large capacity for host manipulation and arsenals that functions during the infection cycle from virus entry to release. The virus infection induces significant morphological changes of host cell that are manipulated to build a virus proliferation factory. Both viruses’ genomes encode a putative nucleoprotein (dinoflagellate/viral nucleoprotein; DVNP), which was also found in the virion proteome of PkV RF02. Phylogenetic analysis suggests that DVNPs are widespread in marine giant metaviromes. Furthermore, the analysis shows that the dinoflagellate homologues were possibly acquired from viruses of the order Imitervirales.
Project description:In contrast to living organisms, viruses lack the machinery necessary for protein synthesis, instead co-opting host factors to translate viral transcripts. Giant DNA viruses, which encode putative orthologs of translation factors, challenge this dogma and blur the line between cellular and acellular biology. Here, we discover a complete mRNA cap-binding complex in mimivirus that we name viral IF4F. This complex associates with host ribosomes to orchestrate the temporally regulated synthesis of structural proteins critical for assembly of viral progeny. The m7G cap-binding subunit of viral IF4F has evolved specificity for viral cap structures through additional interactions with methylated adenosines in mRNAs. Viral IF4F further facilitates dynamic gene expression required to evade host abiotic stresses that normally restrict viral replication. Our study thereby extends a paradigm of eukaryotic translation regulation to include viruses and illuminates a series of evolutionary innovations to a core process of life.
Project description:5-methylcytosine (5mC) is a widespread silencing mechanism that controls genomic parasites. However, in many eukaryotes 5mC has gained complex roles in gene regulation beyond parasite control. Animals are a paradigmatic case for 5mC evolution, as they show widespread variability across lineages, ranging from gene regulation and transposable element control to loss of this base modification. Here we show that the protist animal relative Amoebidium appalachense displays both transposon and gene body methylation, a pattern reminiscent of invertebrates and plants. Unexpectedly, large hypermethylated regions of the Amoebidium genome derive from viral insertions, including hundreds of endogenised giant viruses contributing 14% of the encoded genes. Using a combination of inhibitors and functional genomic assays, we demonstrate that 5mC silences these giant virus insertions. Moreover, alternative Amoebidium isolates show polymorphic giant virus insertions, highlighting a dynamic process of infection, endogenisation and purging. Therefore we propose that 5mC is critical for the controlled co-existence of newly acquired viral DNA into eukaryotic genomes, making Amoebidium a unique model to understand the hybrid origins of eukaryotic genomes.