MRL-1237, [1-(4-fluorophenyl)-2-(4-imino-1,4-dihydropyridin-1-yl) methylbenzimidazole hydrochloride], is a potent and selective inhibitor of the replication of enteroviruses. To reveal the target molecule of MRL-1237 in viral replication, we selected spontaneous MRL-1237-resistant poliovirus mutants. Of 15 MRL-1237-resistant mutants obtained, 14 were cross-resistant to guanidine hydrochloride (mrgr), while 1 was susceptible (mrgs). Sequence analysis of the 2C region revealed that the 14 mrgr mutants contained a single nucleotide substitution that altered an amino acid residue from Phe-164 to Tyr. The mrgs mutant, on the other hand, contained a substitution of Ile-120 to Val. Through the construction of a cDNA-derived mutant, we confirmed that the single mutation at Phe-164 was really responsible for the reduced susceptibility to MRL-1237. MRL-1237 inhibited poliovirus-specific RNA synthesis in HeLa cells infected with a wild strain but not with an F164Y mutant. We furthermore examined the effect of mutations of the 2C region on the drug sensitivity of cDNA-derived guanidine-resistant and -dependent mutants. Two guanidine-resistant mutants were crossresistant to MRL-1237 but remained susceptible to another benzimidazole, enviroxime. Either MRL-1237 or guanidine stimulated the viral replication of two guanidine-dependent mutants, but enviroxime did not. These results indicate that MRL-1237, like guanidine, targets the 2C protein of poliovirus for its antiviral effect.Poliovirus is a member of the family Picornaviridae, containing a positive-sense, single-stranded RNA as a viral genome. The genome encodes a single precursor polyprotein which is eventually cleaved to four structural and seven nonstructural proteins (58). The structural capsid proteins of picornaviruses are located at the amino-terminal P1 region of the polyprotein. The remainder contains viral nonstructural proteins, including two proteases (2A pro and 3C pro ), an RNA-dependent RNA polymerase (3D pol ), and several other proteins essential for viral RNA synthesis. In poliovirus-infected cells, nonstructural proteins 2B, 2C, 3A, 3B, and 3D and their precursors are associated with a specific structure of virus-induced cytoplasmic membranous vesicles, the so-called replication complex (13,22), and implicated in viral RNA synthesis together with viral RNA and some cellular proteins (13,14,17,22,44,61,64,65,73).The 2C protein of picornaviruses contains conserved nucleoside triphosphate (NTP)-binding and RNA helicase motifs in its middle region (19,(28)(29)(30)(31). Genetically manipulated mutations in the NTP-binding motif of the poliovirus 2C protein abolished viral replication and RNA synthesis (47, 70). The replication of viruses with such mutations in the NTP-binding motif was poorly complementated in trans (71). These results indicate the functional significance of the NTP-binding motif in viral replication. A recombinant poliovirus 2C (or precursor 2BC) protein fused with maltose-binding protein (MBP), MBP-2C, was expressed in Escherichia coli and...