The polar alkaline disassembly of papaya mosaic virus.

The disassembly of papaya mosaic virus (PMV) at alkaline pH (pH 10) was monitored by following the decrease of turbidity at 320 nm and by electron microscopy. Most of the virion particle population undergoes a sequential loss of protein subunits starting exclusively from the 3'OH end of the RNA, but a small fraction resists degradation. The progressive decrease in size distribution suggests a sequential process and the presence of "brush-like" structures at one end of the virus particle favors a polar model for disassembly. Furthermore, after treatment with nucleases to digest the exposed RNA tails, the 5' cap structure (m7GpppGp) was found to be conserved in the remaining nucleoprotein intermediates. This indicates that the disassembly of PMV occurs from the 3' terminus to the 5' end, a polarity opposite to that of viral assembly. These RNA fragments (containing the 5' end) retain their ability to reassemble with the viral protein. This result is consistent with earlier evidence that the initiation site for virus assembly is located at the 5' end of PMV-RNA. Electron microscope examination suggests the presence of meta-stable size classes during the disassembly process. This observation might indicate a variability of the RNA-protein interactions along the RNA molecules.