Crystal structures of Norwalk virus polymerase bound to an RNA primer-template duplex and either the natural substrate CTP or the inhibitor 5-nitrocytidine triphosphate have been determined to 1.8 Å resolution. These structures reveal a closed conformation of the polymerase that differs significantly from previously determined open structures of calicivirus and picornavirus polymerases. These closed complexes are trapped immediately prior to the nucleotidyl transfer reaction, with the triphosphate group of the nucleotide bound to two manganese ions at the active site, poised for reaction to the 3-hydroxyl group of the RNA primer. The positioning of the 5-nitrocytidine triphosphate nitro group between the ␣-phosphate and the 3-hydroxyl group of the primer suggests a novel, general approach for the design of antiviral compounds mimicking natural nucleosides and nucleotides.
Norwalk virus (NV)4 is the prototype species of the Norovirus genus within the Caliciviridae and is a major cause of gastroenteritis outbreaks in developed countries (1). Unfortunately, effective treatments are not currently available for many important diseases caused by NV and related RNA viruses. The virally encoded RNA-dependent RNA polymerase (RdRP) is the central enzyme required for replication (2) and is one of the key targets for the development of novel antiviral agents. Recently, 5-nitrocytidine triphosphate (NCT) was identified as a potent inhibitor of picornaviral polymerases, and the nucleoside 5-nitrocytidine was found to have low toxicity and significant antiviral activity in a cultured cell viral infection model (3). A structural and mechanistic basis for rationalizing the inhibitory activity of NCT and related inhibitors is currently lacking because of a shortage of high resolution structural information on RdRP replication complexes.Details on the structure and mechanism of viral RdRPs are clearly required to understand the replication of RNA viruses and to develop more effective antiviral agents. Previous structural studies of viral RdRPs from positive strand RNA viruses and double-strand RNA viruses indicate that the general features of RdRP architecture are highly conserved throughout a diverse range of viruses (reviewed in Refs. 2 and 4). The threedimensional arrangement of N-terminal, fingers, palm, and thumb domains, as well as the active site residues in motifs A-F are nearly universally shared (5).The structural conservation seen in RdRPs suggests that the enzymatic mechanism of nucleotidyl transfer is also highly conserved. Studies primarily on poliovirus RdRP have revealed many of the basic features underlying the nucleotidyl transfer reaction in RdRPs (6, 7). These studies and others indicate that RdRPs, like other polynucleotide polymerases, follow a fivestep reaction cycle involving (i) the binding of an NTP complementary to the base of the template to form an initial "open" complex, followed by (ii) a conformational change to the "closed" complex, (iii) nucleotidyl transfer and translocation, (iv) a second confor...