Mechanism of tobacco mosaic virus assembly: role of subunit and larger aggregate protein.

Tobacco mosaic virus (TMV) was reconstituted from the RNA of a common strain (OM) and the protein of a watermelon strain of cucumber green mottle mosaic virus (CGMMV-W), which is a member of the tobamovirus group. In 0.25 M phosphate buffer at 25 degrees C, CGMMV-W protein existed mainly as 21S aggregates. When this protein was mixed with OM RNA, complexes of short rods were formed but further elongation did not occur. After the addition of subunits in 0.1 M phosphate buffer at 25 degrees C, elongation to the 5' end of the RNA proceeded as fast as in the case of reconstitution with the usual equilibrium "disk preparation" of OM protein, to give 260-nm intermediates in the first 5-7 min. The results proved that the rapid elongation we previously observed in the reconstitution of TMV-OM following the assembly initiation is the outcome of preferential incorporation of TMV subunit protein. Either preformed 21S aggregate or the subunit of CGMMV protein was added to the 260-nm intermediate. Elongation to the 3' end of the RNA was investigated in 0.1 M phosphate buffer at 25 degrees C by measuring the distribution of rod length and the RNase-resistant infectivity. The results showed that the 21S aggregate is kinetically favored as the protein source during the slow elongation process.