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Oat chlorotic stunt virus (OCSV) is a 35 nm icosahedral plant virus comprising of a single capsid protein with a Mr of 48.2 kDa and a 4.1 kb single stranded, positive sense genomic RNA. Northern blot analysis detected a single 3′ terminal subgenomic RNA in extracts from infected plants, which was also found to be encapsidated. Virion RNA directs the synthesis of a 23 kDa polypeptide in a rabbit reticulocyte in vitro translation system. Primer extension analysis has been used to map the end of both the genomic and subgenomic RNA’s, and has shown the genomic size to be 4115 nucleotides in length. The results have enabled a model for the genome expression to be proposed.

During 1992/1993 and 1994/1995, field experiments were carried out to investigate the infection of several cultivars of oats, other cereals and grasses, with oat chlorotic stunt virus (OCSV), which is provisionally identified as a member of the Tombusviridae. Under field conditions a high percentage of oat plants became infected when sown in infested soil and there was little difference in susceptibility between the oat cultivars compared. Although plants of other Poaceae such as winter wheat, winter barley and annual meadow grass also became infected, the percentage was low and the virus titre reached was much lower than in oats. In most infected plants the virus was localized within the roots, and only in a few plants did it spread systemically. This suggests that the virus was acquired through the roots, indicating further similarities between OCSV and members of the Tombusviridae. Several cereals, including oats, wheat, barley and maize, were infected by mechanical inoculation using an embryo-wounding technique; in the majority of cases virus infection only reached a low titre.

The complete genomic sequence of a new virus, first found infecting oats in Wales, UK, has been determined. The genome is a positive-sense ssRNA molecule, 4114 nucleotides in length, examination of which indicates the presence of four ORFs. The first ORF initiating at the 5' terminus (ORF1) encodes a protein with a predicted M r of 23 476 (p23). ORF2 extends through the amber termination codon of ORF1 to give a protein with a predicted M r of 84 355 (p84). The readthrough domain of p84 contains amino acid sequence similarities with a number of putative RNA-dependent RNA polymerases. ORF3 is in a different reading frame from ORF1/2 and encodes a protein with an M r of 48 231 (p48), identified as the coat protein by direct peptide sequencing. ORF4 nests within ORF3 but is in a different frame from it and codes for a protein with a predicted M r of 8220 (p8). Comparisons of peptide sequence, particularly within the putative polymerase region and within the S domain of the coat protein, highlight similarities with members of both the tombusvirus and carmovirus groups. The coat protein region shows most similarity with members of the tombusvirus group, whilst the size and predicted strategy of the genome seem to be intermediate between that of the carmovirus and tombusvirus groups. These features highlight possible evolutionary links with each group whilst being distinct from both. We propose the name of oat chlorotic stunt for this new virus.