The reversible condensation and expansion of the rotavirus genome

Understanding the structural organization of the genome is particularly rel evant in segmented double-stranded RNA viruses, which exhibit endogenous tr anscription activity. These viruses are molecular machines capable of repea ted cycles of transcription within the intact capsid. Rotavirus, a major ca use of infantile gastroenteritis, is a prototypical segmented double-strand ed RNA virus. From our three-dimensional structural analyses of rotavirus e xamined under various chemical conditions using electron cryomicroscopy, we show here that the viral genome exhibits a remarkable conformational flexi bility by reversibly changing its packaging density. In the presence of amm onium ions at high pH, the genome condenses to a radius of approximate to 1 80 Angstrom from approximate to 220 Angstrom, Upon returning to physiologic al conditions, the genome re-expands and fully maintains its transcriptiona l properties. These studies provide further insights into the genome organi zation and suggest that the observed isometric and concentric nature of the condensation is due to strong interactions between the genome core and the transcription enzymes anchored to the capsid inner surface. The ability of the genome to condense beyond what is normally observed in the native viru s indicates that the negative charges on the RNA in the native state may be only partially neutralized. Partial neutralization may be required to main tain appropriate interstrand spacing for templates to move around the enzym e complexes during transcription. Genome condensation was not observed eith er with increased cation concentrations at normal pH or at high pH without ammonium ions. This finding indicates that the observed genome condensation is a synergistic effect of hydroxyl and ammonium ions involving disruption of protein-RNA interactions that perhaps facilitate further charge neutral ization and consequent reduction in the interstrand spacing.