Comparison of the replication properties of murine and human calicivirus RNA-dependent RNA polymerases

The human caliciviruses (CV), norovirus (NoV) and sapovirus (SaV), are major causes of outbreak gastroenteritis worldwide. To date, the investigation of human NoV and SaV replication cycles has been impeded as neither is culturable. Consequently, the recently discovered murine NoV (MNV) has been adopted as a surrogate replication model for the human CVs. In this study, we sought to compare the biochemical properties of the MNV RNA-dependent RNA polymerase (RdRp) with related human NoV and SaV-RdRps to address the suitability of MNV as a model for the human CVs. Three human NoV-RdRps (GII.b, GII.4 and GII.7), an MNV-RdRp and two human SaV-RdRps (GI and GII) were overexpressed in Escherichia coli, purified and their enzymatic activity and fidelity compared. Despite similar to 70% amino acid variation between the RdRp from the two different CV genera, the majority of the physiological characteristics of the RdRps were similar. All RdRps exhibited co-operative dimerisation and had optimal activity at 25A degrees C, a pH range between 7 and 8, required 2-5 mM MnCl2 and were inhibited with increasing NaCl concentrations. We observed RdRp activity at temperatures as low as 5A degrees C and as high as 65A degrees C. Using an in vitro fidelity assay, similar mutation rates were observed for the separate RdRps (1 x 10(-4)-1 x 10(-5)). This is the first report to compare the physiological, biochemical and mutational properties of the MNV-RdRp to those of the human CV-RdRps and it suggests that MNV may be directly applicable to the study of human NoV.