Brome mosaic virus coat protein inhibits viral RNA synthesis in vitro

In vitro, the coat protein of brome mosaic virus (BMV) inhibited RNA synthesis by replicase extracted from BMV-infected barley leaves. The inhibition was due to the interaction of coat protein with BMV RNA. Under the same conditions, no inhibition by TMV coat protein or bovine serum albumin was detected. For the complete inhibition of RNA synthesis in vitro, a coat protein:RNA ratio of 620:1 was required, and their preincubation before addition to the reaction mixture was essential. If the coat protein was added to the reaction mixture during incubation, synthesis continued for a few minutes at the level of the control (omission of coat protein), then decreased gradually, and stopped 6 min after the addition of coat protein. These results suggest that the inhibitory effect of coat protein on RNA synthesis in vitro is due to the interference with the binding site of replicase by partial reassembly of nucleoprotein and that this phenomenon may be a cause of cross protection.