Alterations of the conformation of the RNAs of alfalfa mosaic virus upon binding of a few coat protein molecules.

Structural changes in the single-stranded genome RNAs (RNAs 1, 2 and 3) and the subgenomic coat protein messenger (RNA 4) of alfalfa mosaic virus upon addition of a few coat protein molecules of the virus were investigated by measuring the fluorescent intensity of bound ethidium bromide and by circular dichroism. No effect could be observed in the case of the genome RNAs. However, in RNA 4, which is of much less complexity than the genome RNAs, a reduction of the ethidium bromide binding by 30% was found, whereas the positive molar ellipticity at 265 nm was reduced by 9% upon binding of the coat protein. Both changes point to a reduction of the ordered structure of the RNA. Since the protein is known to bind first at the 3'-terminus of RNA 4 and probably also of the genome RNAs, the conformational changes observed could be those thought to be necessary for replicase recognition in this positive-stranded RNa virus which needs the coat protein for starting an infection cycle.