Intermediates in the disassembly of tobacco mosaic virus at alkaline pH Infectivity, self-assembly, and translational activities.
Disassembly of tobacco mosaic virus at alkaline pH produces a series of discrete, kinetically metastable rods of intermediate length, all containing the original 3'-terminus, together with a "stable fraction" of apparently undegraded virus. The physical properties of these various rods and their protected RNAs were already known and some biological properties are now described. The stable fraction of rods was found to have an infectivity less than 5% of that of the control virus, irrespective of whether or not it had been treated with micrococcal nuclease after exposure to the alkaline conditions. This residual infectivity was probably due to contamination with some remaining undegraded virus. In the wheat germ cell-free protein synthesis system, the RNA from the stable fraction that had not been treated with micrococcal nuclease, and which still contained the 5'-terminal "cap" structure m7G(5')ppp(5')G, was translated indistinguishably from control viral RNA. The RNAs from the nuclease-treated stable fraction and from the other partly stripped particles were translated only with greatly reduced efficiency, but the fragment containing the 2,000 nucleotides from the 3' end of the original RNA yielded disproportionate amounts of a 30,000-dalton polypeptide. Reassembly with the viral coat protein occurred readily only with those fragments of the RNA longer than about 1,400 nucleotides from the 3'-terminus, consistent with earlier evidence that the nucleation site for virus assembly is about 1,000 nucleotides from the 3' end. Paradoxically, however, the RNA from the nuclease-treated stable fraction, which lacks only a few nucleotides from the 5' end of the native viral RNA, would not reassemble. The reason for this is unclear, but change in the secondary structure of the RNA in the assembly initiation region is a possible explanation.
Wilson
T M
TM
Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, England.
Perham
R N
RN
Butler
P J
PJ