Site size of cooperative single-stranded RNA binding by poliovirus RNA-dependent RNA polymerase

The poliovirus RNA-dependent RNA polymerase binds cooperatively to single-stranded RNA. We have determined the minimal RNA-binding site size of the poliovirus polymerase using binding titration with oligonucleotides of increasing length. A dramatic increase in affinity was observed when the length of the oligo(U) increased from 8 to 10 nucleotides (nt), arguing that the minimal size of RNA for polymerase binding is 10 nt. Another increase in affinity seen as the oligo(U) reached 24 nt suggests that a 24-nucleotide RNA can be occupied by two polymerase molecules. Direct binding of wild-type polymerase to oligo(U)12 and oligo(U)24 RNAs showed differences in affinity and cooperativity consistent with this model. The increase in binding affinity seen for oligo(U)10 suggests either that the RNA-binding determinants are widely spaced on the polymerase structure or that a substantial conformational change in the polymerase occurs upon the filling of its RNA-binding site.