Coat protein binds to the 3'-terminal part of RNA 4 of alfalfa mosaic virus.

All four RNAs of alfalfa mosaic virus contain a limited number of sites with a high affinity for coat protein [Van Boxsel, J. A. M. (1976), Ph.D. Thesis, University of Leiden]. In order to localize these sites in the viral RNAs, RNA 4 Tthe subgenomic messenger for coat protein) was subjected to a very mild digestion with ribonucleast T1. The ten major fragments, apparently resulting from five preferential hits, were separated and tested for messenger activity in a wheat germ cell-free system, as well as for the capacity to withdraw coat protein from intact particles. Fragments which stimulated amino acid incorporation were assumed to contain the 5 terminus. Strong evidence was obtained for the location of sites with a high affinity for coat protein near the 3' terminus. The smallest fragment which has the 3'-terminal cytosine comprises only 10% of the length of intact RNA 4 but still possesses these sites. Evidence is presented that the complete coat protein cistron is in the complementing 90% fragment. Possibly, the high-affinity sites are entirely located in the 3'-terminal extracistronic part of RNA 4. They will have the same position in RNA 3 and, possibly, also in the other parts of the genome of alfalfa mosaic virus. The need of this genome for coat protein in order to become infectious may therefore find its explanation in the fact that a conformational change at the 3' ends of the genome parts brought about by the coat protein is required for recognition by the viral replicase.