The site of initiation of rod assembly on the RNA of a tomato and a cowpea strain of tobacco mosaic virus.

Genome RNA of a tomato (T) and a cowpea (Cc) strain of tobacco mosaic virus (TMV), as well as RNA of the short particles of cowpea strain, were tritium labeled at the termini. Chromatographic analyis of the alkali-hydrolysate of these RNAs showed that they all have A(OH) at the 3'-terminus and are capped with m7G ppp at the 5'-terminus. The site of initiation of rod assembly for these strains was determined by sequential reconstitution of virus rods with proteins of two different strains followed by examination of distribution of the proteins on the reconstituted rods by electron microscopic serology. The initiation site on T-RNA was located at the same position as that of a common strain previously studied, being about 800 nucleotides away from the 3'-terminus. In contrast, the initiation site on Cc-RNA was found to be much closer to the 3'-terminus, only about 320 nucleotides away from the terminus, and hence within the coat protein cistron. The results showed that the internal initiation and the bidirectional elongation are a universal mechanism of assembly among TMV strains, but different strains may use different initiation sites. The location of the initiation site of the cowpea strain explained why the coat protein messenger RNA of this strain, but not that of the common and the tomato strains, is encapsidated to form short particles.