An examination of the genomic strategy of pea enation mosaic virus (PEMV) RNA 1 has verified strong organizational and sequence relationships between PEMV and the beet western yellows-potato leafroll luteovirus subgroup. Sequence analysis of RNA 1 demonstrated five predominant open reading frames (ORFs). The extreme 5′ ORF encodes a 34K product of unknown function. The second ORF encodes an 84K product which overlaps 90% of ORF 1 (in a unique reading frame) and is expressed by internal initiation beginning at the second start codon from the 5′ terminus. This protein contains a protease-like motif characteristic of serine- and cysteine-based proteases, suggesting involvement in post-translational processing of viral translation products. The third ORF is characterized by a number of RNA polymerase motifs and a helicase-like motif typical of RNA-dependent RNA polymerases. It overlaps (out of frame) the ORF 2 product and is proposed to be expressed by a frameshift fusion of the ORF 2 and ORF 3 products. The fourth ORF encodes the viral coat protein, and is immediately followed in frame by a 33K ORF thought to represent the aphid transmission subunit of the PEMV virion. Northern blot analysis of polysome-associated RNA suggests that both products are expressed from an 1800 nucleotide subgenomic mRNA, with the 33K product expressed as a read-through fusion with the coat protein monomer.


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