Cloning and sequence analysis of RNA-2 of a mechanically transmitted UK isolate of barley mild mosaic bymovirus (BaMMV).

A mutant of the 'Streatley' isolate of barley mild mosaic bymovirus was selected from the original field isolate by repeated mechanical inoculation. Unlike the wild-type barley mild mosaic virus, which is transmitted by the soilborne fungus Polymyxa graminis, the mutant could not be transmitted by this vector. RNA-2 of the mutant virus was shorter than that of the wild-type virus suggesting that a deletion of part of the genome segment had occurred. The nucleotide sequence of the mutant RNA-2 was determined and revealed a high degree of homology with the RNA-2 of a German BaMMV field isolate. The deletion comprises 1092 nucleotides and is located in the 3'-terminal part of the coding region. The 34-kDa truncated form of the C-terminal protein is less than half the size of the corresponding protein of full-length BaMMV RNA-2. Taken together, the sequence data and results of biological experiments suggest an essential role of the C-terminal protein for fungus transmission of BaMMV.