THE 2ND AMINO-ACID OF ALFALFA MOSAIC-VIRUS COAT PROTEIN IS CRITICAL FOR COAT PROTEIN-MEDIATED PROTECTION

Transgenic plants expressing the coat protein (CP) of alfalfa mosaic virus (AlMV) are resistant to infection by AlMV. A mutation was introduced into the second amino acid of the cDNA for the CP of AlMV. Three different transgenic tobacco lines expressing the mutant CP and two different transgenic tobacco lines expressing the wild-type CP at similar levels were challenged with AlMV virions and viral RNA. Whereas the lines expressing the wild-type CP were highly resistant to infection by AlMV virions and viral RNA, the lines expressing the mutant CP were susceptible to infection by both. The binding affinity of the mutant and the wild-type CPs for the 3' terminal protein binding site on AlMV RNAs was similar, as determined by electrophoretic mobility shift assay. A mixture of AlMV genomic RNAs 1-3 was infectious on the plants expressing the mutant CP but not on vector control plants or plants expressing the wild-type CP, indicating that the mutant CP can activate the AlMV genomic RNAs for infection. These results demonstrate that the second amino acid of the AlMV CP is critical for protection from AlMV but not for the initial interaction between the AlMV RNA and CP, suggesting that this initial interaction does not play a major role in CP-mediated protection.