The nucleotide sequence of the genomic RNA of potato leafroll virus was determined and its genetic organization deduced. The RNA is 5882 nucleotides long and contains 6 open reading frames (ORFs) encoding proteins of 70, 70, 56, 28, 23 and 17 kDa. The putative genes for the coat protein (23 kDa) and the RNA-dependent RNA polymerase (70 kDa) were identified by interviral amino acid sequence homologies. For expression of the different ORFs, translational frameshift and readthrough events are proposed.cDNA cloning; Nucleotide sequence; Amino acid sequence homology; RNA virus, plant positive-strand; (Potato leafroll virus)

An Austrian isolate of potato virus yNTN, the causal agent of potato tuber necrotic ringspot disease (PTNRD), was serologically compared with seven Dutch PVY N isolates. Using polyclonal and monoclonal antibodies, it was found indistinguishable from PVY r~. Determination of the nucleotide sequence of the coat protein cistron and comparison of the deduced amino acid sequence with coat protein sequences of other potyviruses revealed a high level of homology with PVY s coat protein sequences. This confirmed the close taxonomic relationship of PVY Nrs with the PVY s subgroup of potato virus Y. PVY m~ is able to overcome all resistance genes known so far in commercial potato cultivars. Remarkably, transgenic PVY-protected tobacco plants are also resistant to PVY N~ infection upon mechanical and aphid-mediated inoculation. These experiments indicate that genetically engineered resistance offers great potential in protection of potato to new aggressive strains of PVY N.

Transgenic potato plants, cultivar Désirée, were produced that contained the coat protein gene of potato leafroll luteovirus (PLRV). The transformed potato plants expressed the PLRV coat protein (CP) RNA sequences but accumulation of coat protein in transgenic tissues could not be detected. Upon inoculation with PLRV, the PLRV CP RNA expressing potato plants showed a reduced rate of virus multiplication.

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The buoyant densities of bean yellow mosaic virus (BYMV) B25, pea mosaic virus (PMV) E198, lettuce mosaic virus (LMV), and potato virus yN (pVyN) were 1.318, 1.321, 1.330, and 1.326 g/ml, respectively. Their S values were 143, 140, 143, and 145 S. The particle morphology of BYMV B25, PMV E198, and LMV could reversibly be changed by magnesium ions. PVY N particles were broken in the presence of magnesium ions. The molecular weight of the coat protein subunit of the four viruses was 34,000 daltons. In many preparations also a 28,000 daltons component was present. This must be considered to be a breakdown product, derived from the 34,000 daltons component by proteolytic activity.

A new carlavirus, apparently omnipresent in shallot (Allium ascalonicum) without causing symptoms, is described as shallot latent virus. It has also been detected in naturally infected onion (A. cepa) Dilution end-point was 104-10s, thermal inactivation at ca. 80~ and ageing in vitro 8 11 days. Purification by molecular sieving on Sephadex G-200 followed by equilibrium density-gradient centrifuging in CsCI was successful. Sedimentation coefficient was 147.5 S, buoyant density 1.313 g/cm 3 and molecular mass of protein snbunits 23 200 dalton. With the antiserum (titre 1024) distant serological relationships to some carlaviruses were determined. No inclusion bodies were observed with the light microscope. With the electron microscope a high concentration of straight or slightly curved particles with a normal length of 650 to 652 nm could easily be detected in crude sap and in purified preparations.

Bean yellow mosaic virus, pea mosaic virus, lettuce mosaic virus, and potato virus yN were purified by homogenizing and clarifying infected leaves in a mixture of 0.1 M tris-thioglycollic acid buffer pH 9, carbon tetrachloride and chloroform, followed by differential centrifuging applying moderate centrifugal forces.

The role of the open reading frame 0 (ORF0) of luteoviruses in the viral infection cycle has not been resolved, although the translation product (p28) of this ORF has been suggested to play a role in host recognition. To investigate the function of the potato leafroll luteovirus (PLRV) p28 protein, transgenic potato plants were produced containing the ORF0. In the lines in which the ORF0 transcripts could be detected by Northern (RNA) analysis, the plants displayed an altered phenotype resembling virus-infected plants. A positive correlation was observed between levels of accumulation of the transgenic transcripts and severity of the phenotypic aberrations observed. In contrast, potato plants transformed with a modified, untranslatable ORF0 sequence were phenotypically indistinguishable from wild-type control plants. These results suggest that the p28 protein is involved in viral symptom expression. Southern blot analysis showed that the transgenic plants that accumulated low levels of ORF0 transcripts detectable only by reverse transcription-polymerase chain reaction, contained methylated ORF0 DNA sequences, indicating down-regulation of the transgene provoked by the putatively unfavorable effects p28 causes in the plant cell.