PubMed · 9921225
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K Furuta. 1998. [Viral morphology, structure, and classification].. https://pubmed.ncbi.nlm.nih.gov/9921225/
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The phylum Nucleocytoviricota, formerly known as nucleocytoplasmic large DNA viruses (NCLDVs), comprises evolutionarily related viruses with remarkably diverse genome sizes, coding capacities and virion morphologies. These viruses infect hosts across the eukaryotic tree of life, from protists to humans, and are believed to have emerged during the early stages of eukaryotic evolution. How the basic aspects of virus-host interaction have evolved in different lineages and whether they share a conserved infection cycle remain unclear. In this Review, we synthesize the information on the infection cycles of model representatives from the major orders within the phylum, revealing both shared traits and lineage-specific innovations. We compare the information available for the extensively studied poxviruses, asfiviruses, iridoviruses and chloroviruses with insights from the rapidly expanding literature on the mimiviruses, pandoraviruses, marseilleviruses and pithoviruses. We provide an overview of the molecular details underlying the key stages of Nucleocytoviricota infection cycles: entry via membrane fusion, formation of viral factories organized via phase separation, genome replication, virion morphogenesis through a crescent intermediate, and egress. We highlight outstanding questions in the field, unify concepts across traditionally separated research areas, and provide a conceptual framework to guide future cell biology studies on large double-stranded DNA viruses.
Gene silencing is a multifaceted phenomenon leading to propagative down-regulation of gene expression. Gene silencing, first observed in plants containing transgenes, can operate both at the transcriptional and post-transcriptional levels. Silencing effects can be triggered by nuclear transgenes and by cytoplasmic RNA viruses, and it can be propagated between these elements and endogenous plant genes that share sequence homology. Although some aspects of gene silencing are becoming better understood, little is yet known about the relationship between nuclear and cytoplasmic events. Plant DNA viruses-- both the ssDNA geminiviruses and the reverse-transcribing pararetroviruses-- have properties with the potential to initiate gene silencing in the nucleus and in the cytoplasm. Characteristics include production of multiple copies of viral DNA genomes in the nucleus, illegitimate integration of viral DNA into host chromosomes mimicking transgene transformation, and generation of abundant viral RNAs in the cytoplasm. Evidence is emerging that geminiviruses and plant pararetroviruses can interact with the gene silencing system either from introduced DNA constructs or during viral pathogenesis. Some observations suggest there are complex relationships between DNA viral activity, transcriptional and post-transcriptional gene silencing mechanisms. DNA viruses also have properties consistent with an ability to counteract the plant silencing response. In this article, features of plant DNA viruses are discussed in relation to gene silencing phenomena, and the prospects for understanding the interaction between nuclear and cytoplasmic silencing processes.
Recent observations that viruses are very abundant and biologically active components in marine ecosystems suggest that they probably influence various biogeochemical and ecological processes. In this study, the population dynamics of the harmful bloom-forming phytoplankton Heterosigma akashiwo (Raphidophyceae) and the infectious H. akashiwo viruses (HaV) were monitored in Hiroshima Bay, Japan, from May to July 1998. Concurrently, a number of H. akashiwo and HaV clones were isolated, and their virus susceptibilities and host ranges were determined through laboratory cross-reactivity tests. A sudden decrease in cell density of H. akashiwo was accompanied by a drastic increase in the abundance of HaV, suggesting that viruses contributed greatly to the disintegration of the H. akashiwo bloom as mortality agents. Despite the large quantity of infectious HaV, however, a significant proportion of H. akashiwo cells survived after the bloom disintegration. The viral susceptibility of H. akashiwo isolates demonstrated that the majority of these surviving cells were resistant to most of the HaV clones, whereas resistant cells were a minor component during the bloom period. Moreover, these resistant cells were displaced by susceptible cells, presumably due to viral infection. These results demonstrated that the properties of dominant cells within the H. akashiwo population change during the period when a bloom is terminated by viral infection, suggesting that viruses also play an important role in determining the clonal composition and maintaining the clonal diversity of H. akashiwo populations. Therefore, our data indicate that viral infection influences the total abundance and the clonal composition of one host algal species, suggesting that viruses are an important component in quantitatively and qualitatively controlling phytoplankton populations in natural marine environments.