Completion of the mapping of transcription start sites for the five-gene block subgenomic RNAs of Beet yellows Closterovirus and identification of putative subgenomic promoters

Structure and Expression of Large (+)RNA Genomes of Viruses of Higher Eukaryotes

Viral positive-sense RNA genomes evolve rapidly due to the high mutation rates during replication and RNA recombination, which allowing the viruses to acquire and modify genes for their adaptation. The size of RNA genome is limited by several factors, including low fidelity of RNA polymerases and packaging constraints. However, the 12-kb size limit is exceeded in the two groups of eukaryotic (+)RNA viruses – animal nidoviruses and plant closteroviruses. These virus groups have several traits in common. Their genomes contain 5′-proximal genes that are expressed via ribosomal frameshifting and encode one or two papain-like protease domains, membrane-binding domain(s), methyltransferase, RNA helicase, and RNA polymerase. In addition, some nidoviruses (i.e., coronaviruses) contain replication-associated domains, such as proofreading exonuclease, putative primase, nucleotidyltransferase, and endonuclease. In both nidoviruses and closteroviruses, the 3′-terminal part of the genome contains genes for structural and accessory proteins expressed via a nested set of coterminal subgenomic RNAs. Coronaviruses and closteroviruses have evolved to form flexuous helically symmetrical nucleocapsids as a mean to resolve packaging constraints. Since phylogenetic reconstructions of the RNA polymerase domains indicate only a marginal relationship between the nidoviruses and closteroviruses, their similar properties likely have evolved convergently, along with the increase in the genome size.

An Analysis of the Complete Genome Sequence and Subgenomic RNAs Reveals Unique Features of the Ampelovirus, Grapevine leafroll-associated virus 1

Despite being the first closterovirus documented in grapevines (Vitis sp.), the molecular biology of Grapevine leafroll-associated virus 1 (GLRaV-1, genus Ampelovirus, family Closteroviridae) is still in its infancy. In this study, the complete genome sequence of two GLRaV-1 isolates was determined to be 18,731 (isolate WA-CH) and 18,946 (isolate WA-PN) nucleotides (nt). The genome of WA-CH and WA-PN isolates encodes nine putative open reading frames (ORFs) and the arrangement of these ORFs in both isolates was similar to that of Australian and Canadian isolates. In addition to two divergent copies of the coat protein (CP), the genome of GLRaV-1 isolates contain CP-homologous domain in four genes, making the virus unique among Closteroviridae members. The 5' and 3' nontranslated regions (NTRs) of WA-CH and WA-PN isolates showed differences in size and sequence composition, with 5' NTR having variable number of ∼65-nt-long repeats. Using the 5' NTR sequences, a reverse transcription-polymerase chain reaction and restriction fragment length polymorphism method was developed to distinguish GLRaV-1 variants in vineyards. Northern analysis of total RNA from GLRaV-1-infected grapevine samples revealed three subgenomic RNAs (sgRNAs), corresponding tentatively to CP, p21, and p24 ORFs, present at higher levels, with p24 sgRNA observed at relatively higher abundance than the other two sgRNAs. The 5' terminus of sgRNAs corresponding to CP, CPd1, CPd2, p21, and p24 were mapped to the virus genome and the leader sequence for these five sgRNAs determined to be 68, 27, 15, 49, and 18 nt, respectively. Taken together, this study provided a foundation for further elucidation of the comparative molecular biology of closteroviruses infecting grapevines.

Biotechnology Applications of Grapevine Viruses

Plant virus genomes are engineered as vectors for functional genomics and production of foreign proteins. The application of plant virus vectors is of potential interest to the worldwide, multibillion dollar, grape and wine industries. These applications include grapevine functional genomics, pathogen control, and production of beneficial proteins such as vaccines and enzymes. However, grapevine virus biology exerts certain limitations on the utility of the virus-derived gene expression and RNA interference vectors. As is typical for viruses infecting woody plants, several grapevine viruses exhibit prolonged infection cycles and relatively low overall accumulation levels, mainly because of their phloem-specific pattern of systemic infection. Here we consider the biotechnology potential of grapevine virus vectors with a special emphasis on members of the families Closteroviridae and Betaflexiviridae.

Grapevine leafroll-associated virus 3

Grapevine leafroll disease (GLD) is one of the most important grapevine viral diseases affecting grapevines worldwide. The impact on vine health, crop yield, and quality is difficult to assess due to a high number of variables, but significant economic losses are consistently reported over the lifespan of a vineyard if intervention strategies are not implemented. Several viruses from the family Closteroviridae are associated with GLD. However, Grapevine leafroll-associated virus 3 (GLRaV-3), the type species for the genus Ampelovirus, is regarded as the most important causative agent. Here we provide a general overview on various aspects of GLRaV-3, with an emphasis on the latest advances in the characterization of the genome. The full genome of several isolates have recently been sequenced and annotated, revealing the existence of several genetic variants. The classification of these variants, based on their genome sequence, will be discussed and a guideline is presented to facilitate future comparative studies. The characterization of sgRNAs produced during the infection cycle of GLRaV-3 has given some insight into the replication strategy and the putative functionality of the ORFs. The latest nucleotide sequence based molecular diagnostic techniques were shown to be more sensitive than conventional serological assays and although ELISA is not as sensitive it remains valuable for high-throughput screening and complementary to molecular diagnostics. The application of next-generation sequencing is proving to be a valuable tool to study the complexity of viral infection as well as plant pathogen interaction. Next-generation sequencing data can provide information regarding disease complexes, variants of viral species, and abundance of particular viruses. This information can be used to develop more accurate diagnostic assays. Reliable virus screening in support of robust grapevine certification programs remains the cornerstone of GLD management.

The closterovirus-derived gene expression and RNA interference vectors as tools for research and plant biotechnology

Important progress in understanding replication, interactions with host plants, and evolution of closteroviruses enabled engineering of several vectors for gene expression and virus-induced gene silencing. Due to the broad host range of closteroviruses, these vectors expanded vector applicability to include important woody plants such as citrus and grapevine. Furthermore, large closterovirus genomes offer genetic capacity and stability unrivaled by other plant viral vectors. These features provided immense opportunities for using closterovirus vectors for the functional genomics studies and pathogen control in economically valuable crops. This review briefly summarizes advances in closterovirus research during the last decade, explores the relationships between virus biology and vector design, and outlines the most promising directions for future application of closterovirus vectors.

3'-coterminal subgenomic RNAs and putative cis-acting elements of Grapevine leafroll-associated virus 3 reveals 'unique' features of gene expression strategy in the genus Ampelovirus

BACKGROUND

The family Closteroviridae comprises genera with monopartite genomes, Closterovirus and Ampelovirus, and with bipartite and tripartite genomes, Crinivirus. By contrast to closteroviruses in the genera Closterovirus and Crinivirus, much less is known about the molecular biology of viruses in the genus Ampelovirus, although they cause serious diseases in agriculturally important perennial crops like grapevines, pineapple, cherries and plums.

RESULTS

The gene expression and cis-acting elements of Grapevine leafroll-associated virus 3 (GLRaV-3; genus Ampelovirus) was examined and compared to that of other members of the family Closteroviridae. Six putative 3'-coterminal subgenomic (sg) RNAs were abundantly present in grapevine (Vitis vinifera) infected with GLRaV-3. The sgRNAs for coat protein (CP), p21, p20A and p20B were confirmed using gene-specific riboprobes in Northern blot analysis. The 5'-termini of sgRNAs specific to CP, p21, p20A and p20B were mapped in the 18,498 nucleotide (nt) virus genome and their leader sequences determined to be 48, 23, 95 and 125 nt, respectively. No conserved motifs were found around the transcription start site or in the leader sequence of these sgRNAs. The predicted secondary structure analysis of sequences around the start site failed to reveal any conserved motifs among the four sgRNAs. The GLRaV-3 isolate from Washington had a 737 nt long 5' nontranslated region (NTR) with a tandem repeat of 65 nt sequence and differed in sequence and predicted secondary structure with a South Africa isolate. Comparison of the dissimilar sequences of the 5'NTRs did not reveal any common predicted structures. The 3'NTR was shorter and more conserved. The lack of similarity among the cis-acting elements of the diverse viruses in the family Closteroviridae is another measure of the complexity of their evolution.

CONCLUSIONS

The results indicate that transcription regulation of GLRaV-3 sgRNAs appears to be different from members of the genus Closterovirus. An analysis of the genome sequence confirmed that GLRaV-3 has an unusually long 5'NTR of 737 nt compared to other monopartite members of the family Closteroviridae, with distinct differences in the sequence and predicted secondary structure when compared to the corresponding region of the GLRaV-3 isolate from South Africa.