Two concomitant base substitutions in the putative replicase genes of tobacco mosaic virus confer the ability to overcome the effects of a tomato resistance gene, Tm-1.

A resistance-breaking strain of tobacco mosaic virus (TMV), Ltal, is able to multiply in tomatoes with the Tm-1 gene, unlike its parent strain, L. Comparison of the genomic sequences of L and Lta1 revealed two base substitutions resulting in amino acid changes in the 130 and 180 kd proteins: Gln-979 --> Glu and His-984 --> Tyr. To clarify their involvement in the resistance-breaking property of Lta1, the two substitions were introduced into L by an in vitro transcription system to generate a mutant strain, T1. T1 multiplied in Tm-1/Tm-1 tomatoes with symptoms as did Lta1. Two additional mutant strains were constructed, each of which had one base substitution which caused a His-984 --> Tyr change (T2) or a Gln-979 --> Glu change (T3). T3 multiplied in tomato plants and protoplasts with the Tm-1 gene, indicating that the single base substitution is sufficient to overcome the resistance. T2 also multiplied, but its multiplication was greatly decreased. Although no sequence changes were detected in any progeny viruses recovered from plants without the Tm-1 gene, progeny viruses recovered from T2- or T3- inoculated Tm-1/Tm-1 tomatoes contained in most cases viruses with additional second base substitutions. They caused amino acid changes near the mutagenized residues, suggesting that the ability of T3 to overcome the resistance is not the same as that of Lta1. Sequencing of the genomic RNAs of other independently isolated resistance-breaking strains revealed the same two base substitutions found in the Lta1 RNA. These observations suggest that the two concomitant base substitutions, and possibly also the resulting amino acid changes, guarantee successful replication of these TMV strains in tomatoes containing the Tm-1 gene. A strong correlation was found between the ability to overcome the resistance and a decrease in local net charge, suggesting the involvement of an electrostatic interaction between the viral 130 and 180 kd proteins and a putative host resistance factor.