Regions outside of the leucine-rich repeats of flax rust resistance proteins play a role in specificity determination

Multiple alleles controlling different gene-for-gene flax rust resistance s pecificities occur at the L locus of flax. At least three distinct regions can be recognized in the predicted protein products: the Toll/interleukin-1 receptor homology (TIR) region, a nucleotide binding site (NBS) region, an d a leucine-rich repeat (LRR) region. Replacement of the TIR-encoding regio n of the L6 allele with the corresponding regions of L2 or LH by recombinat ion changed the specificity of the allele from L6 to L7. Replacement of the TIR and most of the NBS-encoding region of L10 with the equivalent region of L2 or L9 generated recombinant alleles having a novel specificity. Howev er, replacement of the L10 TIR-encoding region with the TIR-encoding region of L2 gave rise to an allele with no detectable specificity. These data in dicate that non-LRR regions can determine specificity differences between a llelic gene products and that functional specificity involves interactions between coadapted polymorphic regions in the protein products of the allele s. Evidence for the action of diversifying selection on the TIR region is o bserved.