Mutational analysis of the Arabidopsis nucleotide binding site-leucine-rich repeat resistance gene RPS2

Disease resistance proteins containing a nucleotide binding site (NBS) and a leucine-rich repeat (LRR) region compose the largest class of disease res istance proteins. These so-called NBS-LRR proteins confer resistance agains t a wide variety of phytopathogens. To help elucidate the mechanism by whic h NBS-LRR proteins recognize and transmit pathogen-derived signals, we anal yzed mutant versions of the Arabidopsis NBS-LRR protein RPS2. The RPS2 gene confers resistance against Pseudomonas syringae strains carrying the aviru lence gene avrRpt2. The activity of RPS2 derivatives in response to AvrRpt2 was measured by using a functional transient expression assay or by expres sing the mutant proteins in transgenic plants. Directed mutagenesis reveale d that the NBS and an N-terminal leucine zipper (LZ) motif were critical fo r RPS2 function. Mutations near the N terminus, including an LZ mutation, r esulted in proteins that exhibited a dominant negative effect on wild-type RPS2. Scanning the RPS2 molecule with a small in-frame internal deletion de monstrated that RPS2 does not have a large dispensable region. Overexpressi on of RPS2 in the transient assay in the absence of avrRpt2! also led to an apparent resistant response, presumably a consequence of a low basal activ ity of RPS2. The NBS and LZ were essential for this overdose effect, wherea s the entire LRR was dispensable. RPSP interaction with a 75-kD protein (p7 5) required an N-terminal portion of RPSP that is smaller than the region r equired for the overdose effect. These findings illuminate the pathogen rec ognition mechanisms common among NBS-LRR proteins.