Protein 2CATPase of picornaviruses is involved in the rearrangement of host cell organelles, viral RNA replication, and encapsidation. However, the biochemical and molecular mechanisms by which 2C ATPase engages in these processes are not known. To characterize functional domains of 2C ATPase , we have focused on a cysteine-rich motif near the carboxy terminus of poliovirus 2C ATPase . This region, which is well conserved among enteroviruses and rhinoviruses displaying an amino acid arrangement resembling zinc finger motifs, was studied by genetic and biochemical analyses. A mutation that replaced the first cysteine residue of the motif with a serine was lethal. A mutant virus which lacked the second of four potential coordination sites for zinc was temperature sensitive. At the restrictive temperature, RNA replication was inhibited whereas translation and polyprotein processing, assayed in vitro and in vivo, appeared to be normal. An intragenomic second-site revertant which reinserted the missing coordination site for zinc and recovered RNA replication at the restrictive temperature was isolated. The cysteine-rich motif was sufficient to bind zinc in vitro, as assessed in the presence of 4-(2-pyridylazo)resorcinol by a colorimetric assay. Zinc binding, however, was not required for hydrolysis of ATP. 2CATPase as well as its precursors 2BC and P2 were found to exist in a reduced form in poliovirus-infected cells.Picornaviridae is a family of nonenveloped, single-stranded positive-sense RNA viruses. The family is subdivided into six genera: Enterovirus, Rhinovirus, Parechovirus, Cardiovirus, Aphthovirus, and Hepatovirus. The most intensively studied picornavirus is Poliovirus, a member of the genus Enterovirus and the causative agent of poliomyelitis.The mechanism of replication of picornavirus genomes is poorly understood. As the genome enters the host cells, it does not find enzymes specific for RNA-dependent RNA synthesis. Therefore, the genomes of picornaviruses contain sequences that code for several proteins designed to replicate the viral RNA. The genomes of picornaviruses encode one polyprotein that is cleaved by endogenous proteinases into functional proteins (63). The primer-and template-dependent RNA polymerase 3D pol takes a central role in RNA replication. In vitro, the enzyme catalyzes three different types of reactions. First, 3D pol synthesizes its primer by uridylylating the genome-linked protein VPg to VPg-pU(pU) (43) and uses this nucleotidyl protein to initiate chain elongation, a process that has been called protein priming (43,49). Second, 3D pol transcribes the RNA templates, yielding minus-and plus-strand RNA (21, 48). Third, 3D pol unwinds double-stranded RNA during chain elongation (13).Biochemical and genetic evidence indicate that 3D pol is not sufficient for RNA replication and that additional viral (63) and cellular proteins are required (reviewed in reference 65). Little is known about the mechanisms by which accessory proteins participate in RNA replication. Among the virus-encoded...