NUCLEOTIDE-SEQUENCE AND INFECTIVITY OF CUCUMBER MOSAIC-VIRUS STRAIN-BRNA2 INVOLVED IN RESISTANCE BREAKAGE ON COWPEA

A full-length cDNA clone for RNA2 of cucumber mosaic virus strain B (B -CMV) was generated by two-step PCR using B-CMV RNA as a template, and its complete nucleotide sequence was determined. B-CMV RNA2 consists of 3049 nucleotides and contains a single open reading frame encoding 857 amino acids of viral protein 2a. It contains sequence motifs as we ll as conserved sequences within the central region of the 2a gene. Th e RNA transcripts of the cDNA clone for B-CMV RNA2 were shown to be in fectious when inoculated onto tobacco plants in combination with RNAs 1 and 3 of cucumber mosaic virus strain Fny (Fny-CMV). The pseudorecom binant F1B2F3-CMV was inoculated onto cowpea plants to delimit the RNA genome specifying resistance breakage. It infected cowpea systemicall y, producing mosaics on cowpea. Fny-CMV, however, produced a hypersens itive response on cowpea resulting in tiny necrotic local lesions on i noculated leaves, and did not infect cowpea systemically. These result s suggest that RNA2 of B-CMV is involved in resistance breakage on cow pea.