The N gene, a member of the Toll-IL-1 homology region-nucleotide binding site-leucine-rich repeat region (LRR) class of plant resistance genes, encodes two transcripts, N(S) and N(L), via alternative splicing of the alternative exon present in the intron III. The N(S) transcript, predicted to encode the full-length N protein containing the Toll-IL-1 homology region, nucleotide binding site, and LRR, is more prevalent before and for 3 hr after tobacco mosaic virus (TMV) infection. The N(L) transcript, predicted to encode a truncated N protein (N(tr)) lacking 13 of the 14 repeats of the LRR, is more prevalent 4-8 hr after TMV infection. Plants harboring a cDNA-N(S) transgene, capable of encoding an N protein but not an N(tr) protein, fail to exhibit complete resistance to TMV. Transgenic plants containing a cDNA-N(S)-bearing intron III and containing 3' N-genomic sequences, encoding both N(S) and N(L) transcripts, exhibit complete resistance to TMV. These results suggest that both N transcripts and presumably their encoded protein products are necessary to confer complete resistance to TMV.