THE ARABIDOPSIS DOWNY MILDEW RESISTANCE GENE RPP5 SHARES SIMILARITY TO THE TOLL AND INTERLEUKIN-1 RECEPTORS WITH N AND L6

Plant disease resistance genes operate at the earliest steps of pathog en perception. The Arabidopsis RPP5 gene specifying resistance to the downy mildew pathogen Peronospora parasitica was positionally cloned. It encodes a protein that possesses a putative nucleotide binding site and leucine-rich repeats, and its product exhibits striking structura l similarity to the plant resistance gene products N and L6. Like N an d L6, the RPP5 N-terminal domain resembles the cytoplasmic domains of the Drosophila Toll and mammalian interleukin-1 transmembrane receptor s. In contrast to N and L6, which produce predicted truncated products by alternative splicing, RPP5 appears to express only a single transc ript corresponding to the full-length protein. However, a truncated fo rm structurally similar to those of N and L6 is encoded by one or more other members of the RPP5 gene family that are tightly clustered on c hromosome 4. The organization of repeated units within the leucine-ric h repeats encoded by the wild-type RPP5 gene and an RPP5 mutant allele provides molecular evidence for the heightened capacity of this domai n to evolve novel configurations and potentially new disease resistanc e specificities.