The results of the meetings of the International Committee on Taxonomy of Viruses, held in Madrid, September 1975, are briefly reported: rules of viral nomenclature, composition of the new Executive Committee, and a list of the names so far officially agreed. Introduction The International Committee on the Taxonomy of Viruses (ICTV), which is a committee of the Section on Virology of the International Association of Microbiological Societies (IAMS), completed a round of meetings during the Third International Congress for Virology that was held in Madrid from September 10--17, 1975. Since ICTV only meets during these conferences, which are held every four years, the meetings are important occasions for reviewing the classification and nomenclature of viruses. Decisions on new names, which encapsulate the recognition of natural "groups" of viruses, evolve slowly. Official approval for new names depends upon a series of sequential steps ; recommendations by one or more of the subcommittees of the Executive Committee of ICTV (subcommittees on Bacterial, Invertebrate, Plant and Vertebrate Viruses respectively, and for some of the larger viral groups which span several kinds of hos t s the Coordination Subcommittee), which are considered by the Executive Committee of ICTV and may finally be submitted for approval by ICTV itself. Only after this final approval does a name become "official". The results of the last five years of work by ICTV and its committees will be published early this year as a separate volume of "Intervirology", the official journal of the Section on Virology of IAMS, and additional copies will be produced for sale as separates. This "Second P~eport" will include, besides "approved" and common names of virus groups, a brief description of the properties of each

Classification and Nomenclature of Viruses

Despite the rapid mutational change that is typical of positive-strand RNA viruses, enzymes mediating the replication and expression of virus genomes contain arrays of conserved sequence motifs. Proteins with such motifs include RNA-dependent RNA polymerase, putative RNA helicase, chymotrypsin-like and papain-like proteases, and methyltransferases. The genes for these proteins form partially conserved modules in large subsets of viruses. A concept of the virus genome as a relatively evolutionarily stable "core" of housekeeping genes accompanied by a much more flexible "shell" consisting mostly of genes coding for virion components and various accessory proteins is discussed. Shuffling of the "shell" genes including genome reorganization and recombination between remote groups of viruses is considered to be one of the major factors of virus evolution. Multiple alignments for the conserved viral proteins were constructed and used to generate the respective phylogenetic trees. Based primarily on the tentative phylogeny for the RNA-dependent RNA polymerase, which is the only universally conserved protein of positive-strand RNA viruses, three large classes of viruses, each consisting of distinct smaller divisions, were delineated. A strong correlation was observed between this grouping and the tentative phylogenies for the other conserved proteins as well as the arrangement of genes encoding these proteins in the virus genome. A comparable correlation with the polymerase phylogeny was not found for genes encoding virion components or for genome expression strategies. It is surmised that several types of arrangement of the "shell" genes as well as basic mechanisms of expression could have evolved independently in different evolutionary lineages. The grouping revealed by phylogenetic analysis may provide the basis for revision of virus classification, and phylogenetic taxonomy of positive-strand RNA viruses is outlined. Some of the phylogenetically derived divisions of positive-strand RNA viruses also include double-stranded RNA viruses, indicating that in certain cases the type of genome nucleic acid may not be a reliable taxonomic criterion for viruses. Hypothetical evolutionary scenarios for positive-strand RNA viruses are proposed. It is hypothesized that all positive-strand RNA viruses and some related double-stranded RNA viruses could have evolved from a common ancestor virus that contained genes for RNA-dependent RNA polymerase, a chymotrypsin-related protease that also functioned as the capsid protein, and possibly an RNA helicase.

Evolution and Taxonomy of Positive-Strand RNA Viruses: Implications of Comparative Analysis of Amino Acid Sequences

The potyvirus group [named after its type member, potato virus Y (PVY)] is the largest of the 34 plant virus groups and families currently recognized (Ward & Shukla, 1991). It contains at least 180 definitive and possible members (or 30% of all known plant viruses) which cause significant losses in agricultural, pasture, horticultural and ornamental crops (Ward & Shukla, 1991). These viruses are unique in the diversity of inclusion bodies that are formed during the infection cycle (see Lesemann, 1988). A feature shared by all potyviruses is the induction of characteristic pinwheel or scroll-shaped inclusion bodies in the cytoplasm of the infected cells (Edwardson, 1974). These cylindrical inclusion (CI) bodies are formed by a virus-encoded protein and can be considered as the most important phenotypic criterion for assigning viruses to the poty-virus group (Milne, 1988; Shukla et al., 1989; Ward & Shukla, 1991).

Highlights and prospects of potyvirus molecular biology

https://www.cell.com/cell/pdf/0092-8674(88)90031-1.pdf

Signals for ribosomal frameshifting in the Rous sarcoma virus gag-pol region.

The RNA genome of tobacco etch virus (TEV), a plant potyvirus, functions as an mRNA for synthesis of a 346-kilodalton polyprotein that undergoes extensive proteolytic processing. The RNA lacks a normal 5' cap structure at its terminus, which suggests that the mechanism of translational initiation differs from that of a normal cellular mRNA. We have identified a translation-enhancing activity associated with the 144-nucleotide, 5' nontranslated region (NTR) of the TEV genome. When fused to a reporter gene encoding beta-glucuronidase (GUS), the 5' NTR results in an 8- to 21-fold enhancement over a synthetic 5' NTR in a transient-expression assay in protoplasts. A similar effect was observed when the 5' NTR-GUS fusions were expressed in transgenic plants. By using a cell-free translation system, the translation enhancement activity of the TEV 5' NTR was shown to be cap independent, whereas translation of GUS mRNA containing an artificial 5' NTR required the presence of a cap structure. Translation of GUS transcripts containing the TEV 5' NTR was relatively insensitive to the cap analog m7GTP, whereas translation of transcripts containing the artificial 5' NTR was highly sensitive. The 144-nucleotide TEV 5' NTR synthesized in vitro was shown to compete for factors that are required for protein synthesis in the cell-free translation reaction mix. Competition was not observed when a transcript representing the initial 81 nucleotides of the TEV 5' NTR was tested. These results support the hypothesis that the TEV 5' NTR promotes translation in a cap-independent manner that may involve the binding of proteins and/or ribosomes to internal sites within the NTR.

/pdf/cap-independent-enhancement-of-translation-by-a-plant-43wp2poosh.pdf

Cap-independent enhancement of translation by a plant potyvirus 5' nontranslated region.

The nucleotide sequence of the coding region of tobacco etch virus genomic RNA: Evidence for the synthesis of a single polyprotein☆

Biochemical analysis of the capsid protein gene and capsid protein of tobacco etch virus: N-terminal amino acids are located on the virion's surface.

Nucleotide sequence at the 3' terminus of pepper mottle virus genomic RNA: evidence for an alternative mode of potyvirus capsid protein gene organization

The nucleotide sequence of the 3′-terminal portion of the tobacco etch virus (TEV) genome was determined. The 2324-nucleotide sequence represented approximately one-fourth of the TEV genome and included the capsid protein gene and flanking regions. An open reading frame of 2135 nucleotides and an untranslated region of 189 nucleotides adjacent to a polyadenylate tract were identified. The sequence began within an open reading frame, indicating that the initiation codon was upstream of the available sequence data. The sequence of the 20 NH2-terminal amino acids of the TEV capsid protein was established chemically. An identical amino acid sequence, predicted from the nucleotide sequence, was located, commencing at amino acid - 263. These data indicated that maturation of the capsid protein required a post-translational cleavage of a larger protein precursor, with a probable cleavage site between the amino acids glutamine and glycine.

Sequence determination of the capsid protein gene and flanking regions of tobacco etch virus: Evidence for synthesis and processing of a polyprotein in potyvirus genome expression.

Richard F. Allison

Papers

The nucleotide sequence of the coding region of tobacco etch virus genomic RNA: Evidence for the synthesis of a single polyprotein☆

Biochemical analysis of the capsid protein gene and capsid protein of tobacco etch virus: N-terminal amino acids are located on the virion's surface.

Nucleotide sequence at the 3' terminus of pepper mottle virus genomic RNA: evidence for an alternative mode of potyvirus capsid protein gene organization

Sequence determination of the capsid protein gene and flanking regions of tobacco etch virus: Evidence for synthesis and processing of a polyprotein in potyvirus genome expression.