The complete nucleotide sequence of RNA-2 of a fungally-transmitted UK isolate of barley mild mosaic bymovirus and identification of amino acid combinations possibly involved in fungus transmission

The complete nucleotide sequence of RNA-2 of a fungally-transmitted UK isolate of barley mild mosaic bymovirus (BaMMV isolate UK-F) was determined and compared with other published sequences, particularly UK-M, an isolate derived from the same source but which has been mechanically passaged for several years, has a deletion of about 1 kb and cannot be fungally transmitted. From an alignment of the BaMMV RNA-2 encoded protein with that for barley yellow mosaic bymovirus (BaYMV), several regions of consistent homology were identified and extensive searches made for similarities with the proteins of other fungally-transmitted viruses, especially amongst the furovirus capsid readthrough proteins which seem especially prone to deletion and which have already been implicated in fungus transmission. The amino acid combinations ER (glutamic acid-arginine) or QR (glutamine-arginine) were found consistently in all of the viruses. They occurred in positions predicted to be on the outside of the protein, and therefore available for interaction with the fungus vector, and were also within the regions prone to spontaneous deletion. In view of the lack of other structural or sequence homologies, it is suggested that these motifs are strong candidates for involvement in fungus transmission.