Complete nucleotide sequence of RNA 3 from cucumber mosaic virus (CMV) strain O: comparative study of nucleotide sequences and amino acid sequences among CMV strains O, Q, D and Y

Transgenic tobacco lines expressing defective CMV replicase-derived dsRNA are resistant to CMV-O and CMV-Y

Cucumber mosaic virus (CMV) is a tripartite, positive sense RNA virus causing infections and yield losses to many plant species. Here, we generated a construct containing inverted repeat of 1,793 bp fragment of defective CMV replicase gene derived from RNA2 of cucumber mosaic virus strain O (CMV-O). The replicase gene was modified by deleting a 9 bp region between nucleotides 1909-1918. This caused a deletion in the active centre motif of polymerases, producing defective translated product 9 nucleotides shorter than the full length protein. The RNAi construct containing inverted repeat of the defective gene was used to produce transgenic tobacco lines expressing CMV-derived double-stranded RNA via Agrobacterium-mediated transformation. Of the four transgenic lines inoculated with CMV-O or CMV-Y in vitro and ex vivo, three lines (T1, T4 and T5) showed immunity to both strains of CMV as no symptoms were detected, whereas one line (T7) exhibited high resistance with mild symptoms limited to inoculation portions. No virus could be detected in uninoculated new leaves of the transgenic lines after RT-PCR and Dot-immunobinding assay analyses. Small interfering RNAs present in transgenic lines before and after virus challenge indicates that the resistance was acquired through RNA silencing.

Detection, Differentiation and Phylogenetic Analysis of Cucumber Mosaic Virus Isolates from Cucurbits in the Northwest Region of Iran

One hundred and twenty three cucurbit samples with one or more symptoms of leaf mosaic, leaf distortion, fruit mosaic, stunting, mottling and yellowing were collected from several locations in the northwest region of Iran. Screening by double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) with a cucumber mosaic virus (CMV) polyclonal antibody, produced positive reactions from 13 samples. However, none of these positive samples reacted with a CMV subgroup-II (S-II)-specific monoclonal antibody in a triple antibody sandwich (TAS)-ELSIA. When total RNA from the CMV-infected samples was subjected to reverse transcription polymerase chain reaction (RT-PCR) with a pair of primers corresponding to the flanking regions of the virus coat protein (CP) gene, an expected DNA fragment of about 872 bp was amplified from 10 of the 13 isolates. This fragment covered the CP open reading frame (ORF) plus 92 and 123 bp of the 5' and 3' flanking regions, respectively. Restriction analysis with MspI (HpaII) was done on 9 of the PCR products and revealed a previously described CMV subgroup I (S-I) specific profile (537 and 335 bp fragments) for the isolates B13, B23, B5, SH5, SH17, S342 and S337, and an additional fragment, suggestive of combined profiles, was present for B13, SH5 and S342. Two other isolates, SH12 and B7 had a CMV S-II MspI profile (four visible fragments and a predicted non-visible 28-bp fragment on 2% agarose). Also, BsuRI (HaeIII) did not cut the PCR products characteristic of the CMV S-I specific MspI profile, whereas for the S-II isolates, BsuRI gave two fragments with sizes of approximately 559 and 313 bp. Nucleotide (nt) sequences of clones from the isolates B13, B23, SH5, SH17, S337 and SH12 were determined and aligned with those of previously published CMV strains and isolates. Consensus parsimonious trees constructed on the basis of the whole amplified region (841 nt excluding the primer sequences), CP ORF (nt or deduced amino acid data), or either of the flanking regions confirmed the RFLP data so that B13, B23, S337, SH5 and SH17 were placed in the CMV S-IA subclade, and SH12 in the S-II Clade. These analyses showed that both CMV S-I and S-II variants occur in the northwest region of Iran although S-I variants appeared to have a higher incidence.

Phosphorylation of the movement protein of cucumber mosaic virus in transgenic tobacco plants

The 3a protein of Cucumber mosaic virus is essential for the cell-to-cell movement of the viral RNA through plasmodesmata. We have introduced an epitope peptide before the stop codon of the 3a protein and cloned the tagged ORF into a binary vector for Agrobacterium-mediated transformation. The established transgenic tobacco lines produced the 3a protein, which was specifically detected with anti-3a and anti-epitope antisera. Metabolic labeling and subsequent immunoprecipitation revealed that [32P]-orthophosphate was incorporated into the 3a protein. The phosphoamino acid analysis indicated that the 3a protein contained phosphoserine but not phosphothreonine or phosphotyrosine. This is the first demonstration of the 3a protein phosphorylation in planta.

Selection of phage-display peptides that bind to cucumber mosaic virus coat protein

Several discrete peptides that bind specifically to the coat protein of cucumber mosaic virus (CMV) were isolated from a diverse phage library displaying random nonapeptides on the major coat protein VIII. Enrichment was shown by polyclonal phage enzyme linked immunosorbent assay (ELISA) after three rounds of selection. Sequencing of the genes encoding 10 of these peptides revealed an absence of any conserved motifs, although nine of them contained a high proportion of proline residues. Some of the selected peptides were displayed at the N-terminus of thioredoxin and expressed in the cytoplasm of Escherichia coli. Both the phage-displayed and thioredoxin-fusion versions of the peptides could detect purified CMV and CMV present in crude leaf extracts from infected plants. By dot blot analysis, a thioredoxin-peptide fusion could readily detect as little as 5 ng of CMV. The peptides did not bind to other plant viruses. These peptides have been shown to be specific and highly sensitive tools in the detection of CMV and, as well as their diagnostic potential, they could form the basis for a novel disease resistance strategy.

Accumulation kinetics of CMV RNA 3-encoded proteins and subcellular localization of the 3a protein in infected and transgenic tobacco plants

The complete nucleotide sequence of RNA 3 of a Spanish isolate of cucumber mosaic virus (CMV-24) has been determined. The encoded putative cell-to-cell movement protein (3a protein) and the coat protein are 279 and 218 amino acids long, respectively. The 3a protein was expressed in Escherichia coli using the vector pT7-7 and was used to raise an immunoserum. We have followed the time course of accumulation of the 3a protein, in parallel to that of the coat protein, and its subcellular localization as a function of time after CMV-24 infection on tobacco plants. The maximum accumulation level of the 3a protein was reached at early stages of infection, being detected in the cytosolic and the cell wall fractions. At later stages of infection, a decline in accumulation levels of the 3a protein was observed, and the protein was essentially associated with the cell wall fractions. These data were corroborated by immunocytochemistry performed in both infected and 3a-expressing transgenic tobacco plants.

Complete genomic RNA sequences of cucumber mosaic virus strain NT9 from Taiwan

The nucleotide sequences of the RNAs 1, 2, and 3 of the cucumber mosaic virus (CMV) Taiwan isolate NT9 were determined and compared at both the nucleotide and amino acid levels with those of CMV strains Fny, Y, O from subgroup I and strain Q from subgroup II. NT9-CMV has an unique feature at the C-terminus of the 3a protein which contains four extra-amino acids. All three RNAs and their encoded proteins, except 2b, of NT9-CMV share more than 90% identity with those of strains in subgroup I, and 72%-85% identity with Q-CMV. The results indicated the conservation of sequences of CMV derived from different geographical locations.

Complete nucleotide sequence of the RNA 3 from subgroup II of cucumber mosaic virus (CMV) strain: Trk7

The complete nucleotide sequence (2209 nucleotides) of the RNA 3 of the cucumber mosaic virus strain Trk7 was determined. Trk7-CMV possesses two open reading frames which encode the 3a protein (279 amino acids) and the coat protein (218 amino acids). Sequence analysis proved the earlier biological and serological classification of the RNA 3 molecule of Trk7-CMV in subgroup II. A high degree of homology was found in the strains Q and Kin of CMV, while the average similarity between the two different subgroups is much lower.

Infectious in vitro transcripts from amplified cDNAs of the Y and Kin strains of cucumber mosaic virus

Using a method based on the polymerase chain reaction (PCR) with primers that include the phi 10 promoter of bacteriophage T7, we obtained cDNA clones of the three RNA genomes of two different strains of cucumber mosaic virus (CMV; Kin and Y strains) from which infectious in vitro transcripts were generated, and demonstrated that the same primers could be used for amplification of at least two other strains of CMV (O and Py). This method is rapid and requires only limited nucleotide (nt) sequence data (16-18 nt) from the termini of the RNA species. Either viral RNA or unpurified RNA samples from infected plants can be used as template for first-strand cDNA synthesis. For cDNAs of RNA1 and RNA2 of the Y strain, the transcription efficiency was substantially lower than with the Kin strain, unless the primer sequence included transcribed G residues on the 5' side of the viral cDNA, so that the promoter for T7 RNA polymerase resembled more closely the canonical sequence from the bacteriophage T7 phi 10 promoter. The lower specific infectivity of transcripts of the modified cDNAs was more than compensated for by increased transcription efficiency. The possibility that the PCR process may introduce deleterious mutations into the viral cDNA was investigated by re-amplification of a functional cloned cDNA of RNA2: all six cDNA clones of the re-amplified cDNA produced transcripts as infectious as those from the progenitor cDNA.

Construction of full-length cDNA clones of cucumber mosaic virus RNAs 1, 2 and 3: generation of infectious RNA transcripts

Full-length cDNA copies of cucumber mosaic virus (CMV) RNAs 1 and 2 of the Fny strain were constructed from partial cDNA clones and were cloned downstream of bacteriophage T7 promoters. In one pair of clones, transcription proceeded from an unaltered T7 promoter such that in vitro transcripts representing RNAs 1 and 2 contained an additional 17 nucleotides at their 5' termini. In a second pair of clones, the T7 promoter/cDNA junction was altered by oligonucleotide-directed mutagenesis such that the in vitro transcripts contained only an additional G residue at their 5' ends. In addition, a full-length cDNA copy of Fny-CMV RNA 3 was constructed from two overlapping cDNA clones and was cloned downstream of an altered T7 promoter such that the resultant in vitro transcripts also contained only an additional G residue at their 5' ends. In vitro transcripts derived from all clones contained an additional C residue at their 3' ends. In vitro transcripts representing RNAs 1, 2 and 3 which contained an additional residue at each terminus were shown to be infectious together in several hosts of CMV.

Induction of tobacco chlorosis by certain cucumber mosaic virus satellite RNAs is specific to subgroup II helper strains

Two satellite (sat) RNAs of cucumber mosaic virus (CMV), B2- and WL3-sat RNAs, which induce systemic chlorosis on tobacco, were inoculated onto tobacco with a number of CMV strains. Systemic chlorosis was observed only when these satellite RNAs were associated with subgroup II CMV strains. Infection of tobacco with various pseudorecombinants of subgroup I and II CMV strains, together with WL3- or B2-sat RNA, suggests that chlorosis is associated with RNA 2 of subgroup II CMV strains. Chlorosis was not induced when B2- or WL3-sat RNAs were inoculated onto tobacco with tomato aspermy virus. In contrast, the induction of chlorosis on tomato by B1-sat RNA did not show any clear dependence on the subgroup of its CMV helper strain although chlorosis did tend to be more severe in association with subgroup II CMV strains.

Cloning and sequencing of RNA-1 cDNA from cucumber mosaic virus strain O

The complete nucleotide (nt) sequence (3369 nt) of RNA 1 of cucumber mosaic virus strain O (CMV-O) was determined. One open reading frame (ORF; 993 aa) could be deduced from the nt sequence. The homologies of the ORF between CMV-O and CMV-Q or CMV-Fny were calculated to be 85% or 97%, respectively. For CMV-O and CMV-Q, the first one-third of the ORF showed a higher degree of homology (89%), as compared with the other portions (82-85%); the first 224 aa showed more than 93% homology. A comparative study of the three viruses revealed that CMV-O is more homologous to CMV-Fny (subgroup I) [corrected]) than to CMV-Q (subgroup II) [corrected].