Translation of turnip rosette virus RNA in rabbit reticulocyte lysates

An essential fifth coding ORF in the sobemoviruses

The sobemoviruses have one of the smallest of all known RNA virus genomes. ORF1 encodes P1 which plays a role in suppression of silencing and virus movement, ORFs 2a and 2b encode the replicational polyproteins P2a and P2ab, and ORF3 encodes the coat protein. Translation of ORF2a from the genomic RNA is dependent on a leaky scanning mechanism. We report the presence of an additional ORF (ORFx), conserved in all sobemoviruses. ORFx overlaps the 5′ end of ORF2a in the +2 reading frame and also extends some distance upstream of ORF2a. ORFx lacks an AUG initiation codon and its expression is predicted to depend on low level initiation at near-cognate non-AUG codons, such as CUG, by a proportion of the ribosomes that are scanning the region between the ORF1 and ORF2a initiation codons. Mutations that disrupt translation of ORFx in turnip rosette virus prevent the establishment of infection.

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The plant-infecting sobemoviruses have a 4–4.5 kb genome with four know coding ORFs.

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We report an additional ORF (ORFx) that is conserved in all sobemoviruses.

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Translation of ORFx is predicted to depend on leaky scanning and non-AUG initiation.

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Mutations that disrupt translation of ORFx prevent the establishment of infection.

Lucerne transient streak virus RNA and its translation in rabbit reticulocyte lysate and wheat germ extract

Two abundant, encapsidated RNAs of lucerne transient streak virus (LTSV) are the 1.5 x 10(6) molecular weight (Mr) linear RNA-1 and both circular (RNA-2) and linear (RNA-3) forms of a 0.15 x 10(6) Mr viroid-like RNA. Two additional discrete minor RNAs, Mr 0.35 x 10(6) and 0.07 x 10(6), and a heterogeneous mixture of RNAs in the Mr range 0.05 to 1.0 x 10(6) are reported. Principal polypeptides translated from unfractionated LTSV RNA in rabbit reticulocyte lysate were of Mr 105,000 (p105), 78,000 (p78), and 33,000 (p33), the last not easily detected after translation in wheat germ extracts. All apparently are encoded in RNA-1. However, p33, which was precipitated by antibody of LTSV particles and presumably is the major capsid protein, was more readily translated from a smaller, most likely the Mr 0.35 x 10(6), RNA. Partial proteolysis and other tests indicate that p105 has a carboxyl terminal extension of p78 amino acid sequences and that neither shares sequences with p33. No translation products were attributed to RNA-2, RNA-3 or the Mr 0.07 x 10(6) RNA.

A Sobemovirus coat protein gene complements long-distance movement of a coat protein-null Dianthovirus

Red clover necrotic mosaic virus (RCNMV; genus Dianthovirus) and Turnip rosette virus (TRoV; genus Sobemovirus) are taxonomically and ecologically distinct plant viruses. In addition, the two genera differ in the role of coat protein (CP) in cell-to-cell movement. However, both are small icosahedral viruses requiring CP for systemic movement in the host vasculature. Here, we show that the TRoV CP gene is capable of facilitating the vascular movement of a Dianthovirus. Substitution of the RCNMV CP gene with the TRoV CP gene permits movement of the resulting chimeric virus to non-inoculated leaves. RCNMV lacking a CP gene or containing a non-translatable TRoV CP gene do not move systemically. This report introduces the molecular characterization of TRoV and describes the unprecedented complementation of systemic movement function by intergenic complete substitution of a plant virus CP gene.

An encapsidated, subgenomic messenger RNA encodes the coat protein of carnation mottle virus

The translation strategy of carnation mottle virus (CarMV) in vitro has been generally assumed to involve internal initiation events on full-length, genomic RNA (4.3 kb). We suggest that this is, at least in part, incorrect. Encapsidated RNA, fractionated on denaturing sucrose gradients, or total RNA from CarMV-infected leaves, fractionated under non-denaturing conditions, was translated in an mRNA-dependent rabbit reticulocyte cell-free system. Evidence for subgenomic RNAs which encode a polypeptide of Mr 38 000 was found. This product was shown to be related to authentic CarMV coat protein by partial proteolysis with alpha-chymotrypsin and SDS/polyacrylamide-gel electrophoresis.

Detection of genomic-length soybean mosaic virus RNA on polyribosomes of infected soybean leaves

Soybean mosaic virus (SMV)-related RNAs were examined in both polyribosomal and nonpolyribosomal fractions of systemically infected soybean leaves. Viral RNAs were detected by Northern blot hybridization analysis using two cloned SMV-cDNAs representing different regions of the viral genome as hybridization probes. Genomic length SMV-RNA (Mr of 3.3 X 10(6] was found in specific association with EDTA-sensitive polyribosomes of infected leaves, indicating that it functions as a messenger RNA in these cells. A smaller SMV-related RNA (Mr of 1.6 X 10(6] was sometimes detected in the polyribosomal fraction; however, reconstruction experiments indicate that this RNA is a breakdown product of the genomic-length RNA, generated during cell fractionation or RNA extraction. Two other SMV-related RNAs with Mr of 2.0 and 0.78 X 10(6) were sometimes detected in infected cells and were not generated from genomic SMV-RNA or intact virus particles in reconstruction experiments. However, these RNAs were exclusively associated with the EDTA-resistant, nonpolyribosomal fraction of infected cells. These data suggest that genomic-length SMV-RNA is the only viral RNA which is translated in these infected plants.