PLANT-PROTEINS THAT BIND TO THE 3'-TERMINAL SEQUENCES OF THE NEGATIVE-STRAND RNA OF 3 DIVERSE POSITIVE-STRAND RNA PLANT-VIRUSES

The replication of positive-strand RNA plant viruses, which involves b oth virus-encoded and plant-encoded proteins, takes place in two stage s: synthesis of a negative-strand RNA using the genomic positive-stran d RNA as a template and synthesis of progeny positive-strand RNA using the negative-strand RNA as a template. Using gel mobility shift and p hotochemical 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 S'-terminal sequences of the negative-strand RNA of three diverse positive-strand RNA plant viruses. The 32K protein was purifi ed to near homogeneity by chromatography on columns of Macro-prep high Q, heparin-sepharose, single-stranded DNA cellulose and poly(U)-sepha rose. 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 n egative-strand RNA of any of the three viruses, or to the 3'-termini o f the mRNAs of two chloroplast genes, psbA or petD, could be detected. We propose that 3'-terminal negative-strand RNA binding proteins, whi ch may be widespread in the plant kingdom, could be utilised by al lea st three different positive-strand RNA plant viruses for the initiatio n of positive-strand RNA synthesis.