Plant proteins that bind to the 3'-terminal sequences of the negative-strand RNA of three diverse positive-strand RNA plant viruses.

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 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.