The nature of the amino acids in the N-terminal 'DAGX' motif of the coat protein of tobacco vein mottling virus (TVMV) that have a direct effect on aphid transmissibility of the virion were further defined by site-directed mutagenesis. In the first position of the DAGX motif, Asp or Asn are required for aphid transmissibility. In the second position, the nonpolar residue Ala, but not the nonpolar Gly or Val or the polar Thr and Ser, is compatible with transmissibility. In the third position, the small, neutral, nonpolar Gly appears to be critical; even substitution of Ala, with a minimal side-chain, drastically reduces transmissibility. Although the amino acid following the DAG sequence is not highly conserved among potyviruses, the presence of an acidic Glu or Asp residue at this position in the TVMV coat protein drastically reduces or abolishes aphid transmissibility. An attempt was made to test the hypothesis that trypsin cleavage of the N terminus is involved in the aphid inoculation process by destroying a trypsin cleavage site downstream from the DAGX motif. While the predicted decrease in transmission occurred from infected plants, there was no effect on the transmission of purified virus. Of the 23 mutations in the DAGX region of TVMV reported here and previously, only two, substitutions of Lys and Arg for Asp, had a detectable adverse effect other than on aphid transmissibility. These, and perhaps other, residues near the N terminus function in some phase of the TVMV life cycle, in addition to aphid transmission.