Structure and protein composition of the rotavirus replicase particle

To understand the role of viral proteins in the replication of rotavirus RNA, we have characterized the structure of subviral particles (SVPs) that synthesize double-stranded RNA (DS RNA). Pulse-labeling of newly made RNA in infected cells showed that rotavirus DS RNA was synthesized either in single-shell (SS)-like particles or in precursor particles that rapidly mature into SS particles. Experiments using a cell-free system demonstrated that most replicase particles containing newly made DS RNA were of greater density in CsCl than single-shelled (SS) particles. However, this was partly due to the presence of single-stranded RNAs as the treatment of replicase particles with micrococcal nuclease reduced their density to between core particles and SS particles. Electrophoretic analysis indicated that replicase particles, purified by centrifugation on CsCl and glycerol gradients, were similar to SS particles, containing the structural proteins VP1, VP2, and VP6. Rotavirus replicase particles were also found to contain the nonstructural proteins NS34 and NS35 and possible host components. The presence of VP6 in enzymatically active replicase particles suggests that, like transcription, this protein may be required for rotavirus RNA replication.