A single reverse mutation in the 126/183-kda replicase gene of the attenuated tomato strain v-69 of tobacco mosaic virus increases the virus pathogenicity

To determine the roles of the mutations that were earlier found in the structure of the attenuated tomato strain V-69 of tobacco mosaic virus (TMV), the nucleotide sequences of genomic RNAs of its four pathogenic revertants have been identified and analyzed. The comparison of the structures of viral genomes has demonstrated that all revertants studied have a single reverse mutation at position 1654 relative to the parental TMV strain V-69. This reversion has been introduced into the plasmid construction harboring a full-length cDNA copy of the TMV strain V-69 RNA, and the infectious transcripts synthesized in vitro have been tested on TMV-sensitive tomato and tobacco plants. The tests have confirmed that the single reversion at position 1654, which changes the amino acid residue at position 528 of the 126-kDa and 183-kDa proteins of the viral replicase is responsible for the increase in the pathogenicity of revertants compared to the TMV V-69 strain. The mutation at position 1654 of the viral genome may be supposed to be the main factor of the attenuation of TMV strain V-69.