A functional ribonucleoprotein complex forms around the 5′ end of poliovirus RNA

The existence of a computer-predicted cloverleaf structure for the first 100 nucleotides at the 5′ end of poliovirus RNA was verified by site-directed mutagenesis and by chemical and RNAase probing. Mutations that modified the cloverleaf in the positive strand but not the negative strand were lethal to the virus. This RNA cloverleaf structure binds a cellular protein and the viral proteins 3Cᵖʳᵒ and 3Dᵖᵒⁱ. Mutations in specific regions of the RNA cloverleaf prevented this binding. Mutations in either 3Cᵖʳᵒ or the RNA that disrupted ribonucleoprotein complex formation inhibited virus growth and selectively affected positive strand RNA accumulation. Phenotypic reversion of these mutations restored the ability to form the complex. Thus, a cloverleaf structure in poliovirus RNA plays a central role in organizing viral and cellular proteins involved in positive strand production.