TOBAMOVIRUS HELPER SPECIFICITY OF SATELLITE TOBACCO MOSAIC-VIRUS INVOLVES A DOMAIN NEAR THE 5' END OF THE SATELLITE GENOME

The molecular basis of the interactions between plant virus satellites and their helper viruses is not understood. The features of the satel lite tobacco mosaic virus (STMV) genome that determine tobamovirus hel per specificity were investigated using two independent strategies. Th e first tested the possible significance of regions of nearly identica l sequence within the 3'-terminal 150 bases of the genomes of STMV and its natural helper virus, tobacco mild green mosaic virus (TMGMV). A chimeric STMV clone containing the 3' terminus of tobacco mosaic virus (TMV-U1) RNA was infectious in coinfections with TMGMV, but it did no t replicate with TMV-U1. In the second strategy, populations of STMV a dapted to replication with four alternative helper tobamoviruses were generated by serial passage in tobacco. RNase protection analyses of t hese RNA populations showed that in all cases there had been a genetic change 50 to 60 bases from the 5' terminus of the STMV genome. Simila r changes were detected in several progenies of STMV clones replicated with TMV-U1, indicating that change at this site was essential for re plication with a helper virus other than TMGMV. Sequence analyses of t he changes at this 'helper adaptation domain' showed consistently the deletion of a single G from five consecutive Gs at bases 61 to 65. Inf ectivity experiments with STMV clones containing this G deletion showe d that this change alone was not sufficient to modify helper specifici ty, so additional factors which remain to be identified must also be i nvolved. Nevertheless, these experiments show the ability of STMV popu lations to undergo rapid, reproducible evolution by both selection of pre-existing variants and de novo mutation, and constitute the first m olecular demonstration that the helper virus acts as a selection press ure on the evolution of satellite populations.