The disassembly of tobacco mosaic virus at alkaline pH is a polar process, sequential loss of protein subunits occurring predominantly, if not exclusively, from the end of the virus particle that contains the 5′-end of the RNA, although approximately 25% of the virus population generally resists degradation. A series of stable intermediates in the disassembly of the labile fraction has been identified, following their separation by sucrose density gradient centrifugation. The ultimate intermediate is one-sixth the length of the original virus, but four larger and less stable fragments were detected. From their sedimentation behavior, the five intermediates were calculated to have rod lengths of 198, 114, 87, 69, and 48 nm, respectively. Direct observation of negatively stained samples in the electron microscope indicated lengths of 195, 115, 92, 66, and 51 nm for the corresponding particles. The agreement is good and suggests that the method used to calculate rod lengths from sedimentation coefficients is reliable. Examination of the RNA remaining within the nucleoprotein fragments confirms that they are reasonably homogeneous. Attempts using sodium dodecyl sulfate-polyacrylamide gel electrophoresis to detect a plugging protein in the stable fraction of the virus population that resists degradation were unrewarded although the presence of such a molecule may have been obscured by the large amounts of coat protein. An extra stability conferred by binding of certain cations is a likely alternative explanation. The existence of intermediates in the alkaline disassembly of the virus is attributed to variability in the interaction between the RNA and the coat protein subunits along the virus rod. It is possible that this property is reflected in the process by which the protein subunits are removed in vivo during infection. © 1978.
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