3CDpro(3CproH40A) enhances the specific infectivity of virus particles produced in vitro. Translation-RNA replication reac tions were carried out in the presence of [35S]TransLabel, as described in Materials and Methods. Where indicated purified 3CDpro(3CproH40A) or 3CDpro(3CproH40G, 3DpolR455A/R456A) protein (5.5 nM) was added to the reactions at t = 0 hr and the samples were incubated for 16 hr at 34°C. Following RNase treatment and dialysis, 0.1% of SDS was added to the samples, as indicated. They were loaded on a 5–20% sucrose gradient (Materials and Methods) and centrifuged for 80 min at 40,000 RPM in a SW41 rotor at 4°C. (A) the 80S peak is shown; (B) the 155S peak is shown.