The nucleotide sequence of the genomic RNA of narcissus mosaic virus (NMV) was deduced from a set of cDNA clones and by direct sequencing of RNA. The genome, with a length of 6955 nucleotides [excluding the 3′poly(A) tail], contains six open reading frames (ORFs) with the capacity to code for polypeptides of more than 10K, with M r values of 186284, 25845, 13998, 11059, 26097 and 10519. The first five of these putative proteins show considerable homology to similar proteins encoded by the RNAs of potato virus X (PVX, 6435 nucleotides) and white clover mosaic virus (WClMV, 5845 nucleotides). The sixth ORF is completely overlapped by the M r 26097 coat protein cistron and has some homology with a similar ORF in WCIMV RNA. The difference in length of the RNAs of NMV, PVX and WCIMV is due to a non-homologous domain of variable length in the central region of the 5′-proximal ORF of the three viruses.
Mutants of African cassava mosaic virus containing extensive deletions across the coat protein gene that remove up to one-third of the genomic component have been constructed and shown to be infectious when mechanically inoculated onto Nicotiana benthamiana by leaf abrasion. Using N. tabacum protoplasts we demonstrate that mutant pCLV. CPΔ11, containing a 712 bp deletion, is competent for replication in its deleted form. However, systemic spread of pCLV. CPΔ11 and other deletion mutants is associated with reversion of DNA 1 to a size comparable to that of the native genomic component. This contrasts with the behaviour of coat protein mutants of the closely related geminivirus tomato golden mosaic virus which maintain their deletions during spread. Appraisal of the different inoculation procedures used to introduce the mutants into plants suggests the imposition of a stringent size requirement for localized cell-to-cell spread which is relaxed for long distance spread through the vascular system.
Three spontaneous mutants of strain TpM-34 of red clover necrotic mosaic virus, designated M-A, M-B and M-C, were isolated. In gel diffusion tests the three mutants were serologically indistinguishable from the parent but when inoculated onto Vigna unguiculata each induced characteristic symptoms which differed from those induced by TpM-34. TpM-34 induced similar numbers of lesions at 17 °C and 26 °C. However, although inoculation with RNA of the mutants induced 70 to 80% as many lesions as inoculation with TpM-34 RNA at 17 °C, very few lesions were induced by any of the mutants at 26 °C. The symptoms induced by heterologous mixtures of RNA 1 or RNA 2 of TpM-34 with RNA 2 or RNA 1 of each mutant indicated that M-A, M-B and M-C each has mutations in both RNA components, but that symptom expression was determined predominantly by RNA 1.
The complete nucleotide sequence (2217 residues) of RNA 3 of cucumber mosaic virus strain O (CMV-O) was determined. Two open reading frames were identified, encoding a 3A protein (279 amino acid residues) in the 5′-proximal region and a coat protein (218 amino acid residues). The amino acid sequence of the coat protein C terminus was determined directly from purified protein, and confirmed the presence of the coat protein open reading frame in CMV-O RNA 3. Comparison of nucleotide sequences and amino acid sequences of CMV strains O, Q, D and Y indicated the close relationship between these strains. A tRNA-like structure could be adopted by the 3′ non-coding region, and this resembled a similar structure in CMV-Q in spite of nucleotide substitutions or deletions.