Structural Transitions in Southern Bean Mosaic Virus and their Correlation with Infectivity and Ribonuclease Sensitivity

Southern bean mosaic virus (SBMV) virions swell when the capsid‐associated divalent cations are removed with EDTA at pH 7.5, resulting in an approximate 99% decline in the specific infectivity. Recompacting swollen virions either with divalent cations at pH 7.5, or by lowering the pH to 5.0 (in the absence or in presence of divalent cations) fails to restore complete infectivity. In contrast to swollen SBMV, RNA within the recompacted virions is fully protected from ribonuclease attack. Removing divalent cations with EDTA at pH 5.0 causes no infectivity loss or conformational change. These results indicate that if SBMV conformation is altered once then an irreversible loss in the infectivity occurs and the divalent cations play no role per se , in the infection process. Furthermore, observations based upon the sedimentation behaviour of ribonuclease‐treated SBMV indicate that RNA must be physically intact for capsid recompaction to occur. Obviously, structural rearrangements at the capsidsurface (e.g., regeneration of intersubunit interactions) and at the virion interior (i.e., RNA‐protein linkages) are involved collectively in conferring conformational stability to the recompacted SBMV.