Bamboo Mosaic Potexvirus Satellite RNA (satBaMV RNA)-encoded P20 Protein Preferentially Binds to SatBaMV RNA

A satellite RNA of 836 nucleotides [excluding the poly(A) tail] depends on the bamboo mosaic potexvirus (BaMV) for its replication and encapsidation. The BaMV satellite RNA (satBaMV) contains a single open reading frame encoding a 20-kDa nonstructural protein (P20). The P20 protein with eight histidine residues at the C terminus was overexpressed in Escherichia coli. Experiments of gel retardation, UV cross-linking, and Northwestern hybridization demonstrated that purified P20 was a nucleic-acid-binding protein. The binding of P20 to nucleic acids was strong and highly cooperative. P20 preferred binding to satBaMV- or BaMV-related sequences rather than to nonrelated sequences. By deletion analysis, the P20 binding sites were mainly located at the 5' and 3' untranslated regions of satBaMV RNA, and the RNA-protein interactions could compete with the poly(G) and, less efficiently, with the poly(U) homopolymers. The N-terminal arginine-rich motif of P20 was the RNA binding domain, as shown by in-frame deletion analysis. This is the first report that a plant virus satellite RNA-encoded nonstructural protein preferentially binds with nucleic acids.