Targeted isolation, sequence analysis, and physical mapping of nonTIR NBS-LRR genes in soybean

Most cloned plant disease resistance genes (R-genes) code for proteins belonging to the nucleotide binding site (NBS) leucine-rich repeat (LRR) superfamily. NBS-LRRs can be divided into two classes based on the presence of a TIR domain ( Toll and interleukin receptor-like sequence) or a coiled coil motif (nonTIR) in their N-terminus. We used conserved motifs specific to nonTIR-NBS-LRR sequences in a targeted PCR approach to generate nearly 50 genomic soybean sequences with strong homology to known resistance gene analogs (RGAs) of the nonTIR class. Phylogenetic analysis classified these sequences into four main subclasses. A representative clone from each subclass was used for genetic mapping, bacterial artificial chromosome (BAC) library screening, and construction of RGA-containing BAC contigs. Of the 14 RGAs that could be mapped genetically, 12 localized to a 25-cM region of soybean linkage group F already known to contain several classical disease resistance loci. A majority of the genomic region encompassing the RGAs was physically isolated in eight BAC contigs, together spanning more than 1 Mb of genomic sequence with at least 12 RGA copies. Phylogenetic and sequence analysis, together with genetic and physical mapping, provided insights into the genome organization and evolution of this large cluster of soybean RGAs.