A conformational switch at the 3' end of a plant virus RNA regulates viral replication.

3' untranslated regions of alfamo- and ilar-virus RNAs fold into a series of stem-loop structures to which the coat protein binds with high affinity. This binding plays a role in initiation of infection ('genome activation') and has been thought to substitute for a tRNA-like structure that is found at the 3' termini of related plant viruses. We propose the existence of an alternative conformation of the 3' ends of alfamo- and ilar-virus RNAs, including a pseudoknot. Based on (i) phylogenetic comparisons, (ii) in vivo and in vitro functional analyses of mutants in which the pseudoknot has been disrupted or restored by compensatory mutations, (iii) competition experiments between coat protein and viral replicase, and (iv) investigation of the effect of magnesium, we demonstrate that this pseudoknot is required for replication of alfalfa mosaic virus. This conformation resembles the tRNA-like structure of the related bromo- and cucumo-viruses. A low but specific interaction with yeast CCA-adding enzyme was found. The existence of two mutually exclusive conformations for the 3' termini of alfamo- and ilar-virus RNAs could enable the virus to switch from translation to replication and vice versa. The role of coat protein in this modulation and in genome activation is discussed.