Expression of two soybean resistance gene candidates shows divergence of paralogous single copy genes

The cloning of several plant genes directly involved in triggering a diseas e resistance response has shown that numerous resistance genes in the nucle otide binding site (NBS)/leucine-rich repeat (LRR) class have similar conse rved amino acid sequences. In this study, we used a short soybean DNA seque nce, previously cloned based on its conserved NBS, as a probe to identify f ull-length resistance gene candidates. Two homologous, but genetically inde pendent genes were identified. One gene maps to the soybean molecular linka ge group (MLG) F and a second is coded on MLG E. The first gene contains a 3,279 nucleotide open reading frame (ORF) sequence and possesses all the fu nctional motifs characteristic of previously cloned NBS/LRR resistance gene s. The N-terminal sequence of the deduced gene product is highly characteri stic of other resistance genes in the subgroup of NBS/LRR genes which show homology to the Toll/Interleukin-1 receptor genes. The C-terminal region is somewhat more divergent as seen in other cloned disease resistance genes. This region of the F-linked gene contains an LRR region that is characteriz ed by two alternatively spliced products which produce gene products with e ither a four-repeat or a ten-repeat LRR. The second cloned gene that maps t o soybean MLG E contains 1,565 nucleotides of ORF in the N-terminal domain. Despite strong homology, however, the 3' region of this gene contains seve ral in-frame stop codons and apparent frame shifts compared to the F-linked gene, suggesting that its functionality as a disease resistance gene is qu estionable. These two disease resistance gene candidates are shown to be cl osely related to one another and to the members of the NBS/LRR class of dis ease resistance genes.