Evidence for pH-induced release of RNA from belladonna mottle virus and the stabilizing effect of polyamines and cations

The RNA of belladonna mottle virus (BDMV) is released from virions by increasing the pH to neutrality or above, leaving behind intact protein shells. In contrast, much harsher conditions are required to release the RNA from the closely related turnip yellow mosaic virus (TYMV). The heat-induced or pH-dependent escape of RNA from BDMV has been investigated by 31P NMR spectroscopy and ultracentrifugation. The methods show a transition pH near 6.8 at which the RNA undergoes a structural alteration probably caused by disruption of protein-RNA linkages. Addition of cations or polyamines to BDMV prevents the RNA loss at alkaline pH; the virions become stable to pH values up to 11.5 as observed for TYMV. We suggest that repulsion of the negatively charged phosphate groups of the nucleic acid provides the driving force for RNA release at pH values above the threshold point where protein-RNA interactions are broken. The polyamines effectively counter the phosphate charge in BDMV and thereby prevent RNA loss. Since TYMV is packaged with polyamines and BDMV is not (R. Virudachalam, K. Sitaraman, K. L. Heuss, P. Argos, and J. L. Markley, Virology 130, 360-371, 1983), the different conditions required for RNA escape from the two viruses are explained. Heating of BDMV virions at pH 7.0 to 40 degrees resulted in their partial disruption; the product some intact particles, empty capsids, and RNA fragments with a sedimentation coefficient of 5S.