In this study, we are examining the mechanism of poliovirus RNA replication. A soluble RNA dependent RNA polymerase has been isolated from poliovirus infected cells. This enzyme was originally discovered and purified by following its activity as a poly(U) polymerase. The purified enzyme will also copy poliovirion RNA when oligo(U) is used as a primer. Under the appropriate in vitro conditions, the purified enzyme will synthesize full-sized copies of a virion RNA template in about ten minutes. Studies on the structure of purified polymerase show that a single virus-specific protein with a molecular weight of about 63,000 (p63) copurifies with the polymerase. This protein was able to specifically initiate synthesis at the 3'-end of a virion RNA template using oligo(U) as a primer and was able to carry out an elongation reaction in which a full-sized RNA product was synthesized. Studies are now underway to determine how poliovirus RNA synthesis initiates. We are now attempting to purify and characterize an activity that has been shown to initiate RNA synthesis in the absence of oligo(U). In addition, we are trying to determine if the protein that is covalently linked to the 5'-end of polivirus RNA (i.e., VPg) is required to initiate RNA synthesis. We ultimately hope to identify all of the components of the replication system that are required to initiate the synthesis of poliovirus RNA.