High resolution analysis of the readthrough domain of beet necrotic yellow vein virus readthrough protein: a KTER motif is important for efficient transmission of the virus by Polymyxa betae.

The 5'-terminal cistron of beet necrotic yellow vein furovirus RNA 2 encodes the 21 kDa major viral coat protein and terminates with an amber stop codon which can undergo suppression to give rise to a 75 kDa readthrough (RT) protein referred to as P75. P75 is a minor component of virions and the 54 kDa RT domain following the coat protein sequence is important both for virus assembly and transmission by the fungal vector Polymyxa betae. To better define the regions of the RT domain involved in these two steps, RNA 2 transcripts encoding different in-frame RT domain deletion mutants were tested for their ability to form virions when inoculated to plants with the other viral RNAs and to be fungus-transmitted. All deletions in the N-terminal half of the RT domain interfered with virus assembly and partially or completely inhibited fungus transmission. A 4 1 1 nucleotide deletion within the C-terminal half of the RT domain did not inhibit assembly but blocked fungus transmission of the virus. Alanine scanning mutagenesis within the aforesaid 4 1 1 nucleotide subdomain identified a peptide motif (KTER) which is important for the fungus transmission process.