The replication of positive-strand RNA plant viruses, which involves both virus-encoded and plant-encoded proteins, takes place in two stages: synthesis of a negative-strand RNA using the genomic positive-strand RNA as a template and synthesis of progeny positive-strand RNA using the negative-strand RNA as a template. Using gel mobility shift and photochemical crosslinking assays, we have identified three proteins of M r 32K, 50K and 100K in extracts of tobacco and spinach leaves that bind to the 3′-terminal sequences of the negative-strand RNA of three diverse positive-strand RNA plant viruses. The 32K protein was purified to near homogeneity by chromatography on columns of Macro-prep high Q, heparin-sepharose, single-stranded DNA cellulose and poly(U)-sepharose. No binding of any of the three proteins to the 3′-termini of the positive-strand RNA or the 5′-termini of the positive-strand RNA or negative-strand RNA of any of the three viruses, or to the 3′-termini of the mRNAs of two chloroplast genes, psbA or petD, could be detected. We propose that 3′-terminal negative-strand RNA binding proteins, which may be widespread in the plant kingdom, could be utilised by at least three different positive-strand RNA plant viruses for the initiation of positive-strand RNA synthesis.
