Replacement of Gln-979 by Glu in the putative replicase of tomato mosaic tobamovirus is sufficient to overcome Tm-1 resistance in tomato. It has been suggested that this change decreases the local net charge of the protein which is important in overcoming the resistance. In this study, we constructed five mutants, designated TLAsn, TLAsp, TLHis, TLLys and TLArg, in which Gln-979 was replaced by Asn, Asp, His, Lys and Arg, respectively, and analysed their abilities to overcome Tm-1 resistance. Unexpectedly, not only TLAsp, but also TLLys multiplied in tomato cells with the Tm-1 gene. TLAsn and TLArg multiplied at a reduced level. Multiplication of TLHis was virtually almost inhibited. From these results, it is unlikely that a decrease in the local net charge is the major reason for overcoming Tm-1 resistance.