Molecular characterisation of potexviruses isolated from three different genera in the family Cactaceae.

The genome properties of three potexviruses which previously had been isolated from different genera in the family Cactaceae and had been found to be only distantly related serologically have been studied. The sequence of the 3040 3' terminal nucleotides of the genomic RNA of isolate K11 from Schlumbergera bridgesii and the complete RNA sequences of isolates B1 and CC10 from Zygocactus sp. and Opuntia sp., respectively, were determined. Starting sequences were obtained by means of immunocapture reverse transcription PCR using primers derived from highly conserved sequences in other potexviral RNAs. The known parts of the sequences were extended by means of random-primed cDNAs and specific primers derived from the known parts of the sequences. The genome structure of the three viruses resembles that of other potexviruses. The conserved motifs typical for replication-associated proteins, triple gene block (TGB) proteins and coat proteins of potexviruses were readily identified in the translation products of the five open reading frames. The 3' untranslated regions of the three RNAs are folded into secondary structures containing three characteristic hairpins. Rather low percentages of amino acid sequence identities ranging from 62% to 76% for the coat proteins and 41% to 49% for TGB proteins 3 suggest that these viruses should be regarded as distinct virus species for which the names Zygocactus virus X, Schlumbergera virus X and Opuntia virus X are proposed. It is also suggested that the name Cactus virus X which originally was coined for all three virus isolates should no longer be used.