No abstract

A spontaneous red clover necrotic mosaic virus mutant, TpM-341, was isolated by multiple passage of Czechoslovakian isolate TpM-34 in beans, followed by three cycles of single lesion isolation in cowpea and in Chenopodium quinoa. The symptoms induced in cowpea by TpM-34 and TpM-341 differed. TpM-34 gave rise to chlorotic lesions which expanded with time, often becoming confluent with adjacent lesions, and developed necrotic margins; the plants became systemically infected. TpM-341 induced necrotic lesions which, once developed, did not expand further; plants did not become systemically infected. Analysis of pseudorecombinants formed between the RNAs of TpM-34 and TpM-341 showed that RNA 2 determined the difference in symptoms. Comparison of the nucleotide sequence of the open reading frames (ORFs) encoding the P2 movement proteins of the two isolates revealed only one difference, a deletion of an A residue in a sequence of four A residues (nucleotides 790 to 793). Construction of full-length TpM-34 and TpM-341 RNA 2 cDNA clones, from which infectious RNA 2 could be transcribed in vitro, and in vitro mutagenesis of a cDNA clone of TpM-341, confirmed that the difference in the symptoms induced by TpM-341 was caused by the loss of this A residue. This single base deletion was predicted to cause a translational frameshift in the P20RF causing the loss of 88 amino acids at the C terminus which were replaced by a sequence of 34 different amino acids, producing a truncated P2 protein. In vitro translation ofRNA 2 transcribed from cDNA clones showed that RNA with four or three A residues starting at nucleotide 790 produced proteins of Mr 36K and 30K respectively, in agreement with the predictions based on the nucleotide sequence.

The complete nucleotide sequence (1448 nucleotides) of RNA 2 of a Czechoslovakian isolate TpM-34 of red clover necrotic mosaic virus (RCNMV-TpM-34) has been determined. The sequence contained one major open reading frame (ORF) with the potential to encode a protein of 326 amino acids (Mr 35755), designated P2. The nucleotide sequence of RNA 2 of RCNMVTpM-34 and the previously published sequence of RNA 2 of an Australian isolate of the virus (RCNMVAus) were 83 % identical and there was 80% amino acid sequence identity between the P2 proteins of these isolates. However the N-terminal two-thirds of the P2 proteins shared a higher degree of similarity than the C-terminal regions which were predicted to have a more flexible structure. An ORF in the 3' portion of RNA 2 of RCNMV-Aus, which could encode a protein of Mr 5000, was not present in RNA 2 of RCNMVTpM-34. RNAs 1 and 2 of RCNMV-TpM-34 and RCNMV-Aus are bilaterally compatible.

Viral diseases are part of the limiting factors to tomato (Solanum lycopersicum L.) cultivation worldwide, reducing both the quality and quantity of yield. Tomato mosaic virus (ToMV) is one of the damaging viruses of tomato. This paper describes molecular characteristics of the full length genome of ToMV isolated from tomato in Uganda (ToMV-Ug). The genomic, ribonucleic acid (RNA), of this isolate is 6383 nucleotides (nts) in length, encoding four open reading frames (ORFs). Based on the homology with other ToMV strains, the 5' proximal 130 kilo dalton (kDa) ORF and its read-through product (180 kDa) are expected to encode two proteins required for viral genome replication; while the 30 kDa middle ORF and the 17.5 kDa 3' proximal ORF are expected to encode the movement protein (MP) and coat protein (CP), respectively. The 5'-and 3'-untranslated regions (UTRs) are 71 and 201 nts, respectively. Comparison with previously published ToMV sequences showed that ToMV-Ug is 99% identical to ToMV strains from Africa (Egypt and Zimbabwe), as well as diverse locations such as China, Australia, Germany and Japan; suggesting high levels of sequence conservation within this virus. This is the first report detailing molecular analysis of a ToMV isolate from Uganda and the Eastern and Central Africa regions.