Expression and genome organization of resistance gene analogs in soybean

Sequence analysis of cloned plant disease-resistance genes reveals a number of conserved domains. Researchers have used these domains to amplify analo gous sequences, resistance gene analogs (RGAs), from soybean and other crop s. Many of these RGAs map in close proximity to known resistance genes. Whi le this technique is useful in identifying potential disease resistance loc i, identifying the functional resistance gene from a cluster of homologs re quires sequence information from outside of these conserved domains. To stu dy RGA expression and to determine the extent of their similarity to other plant resistance genes, two soybean cDNA libraries (root and epicotyl) were screened by hybridization with RGA class-specific probes. cDNAs hybridizin g to RGA probes were detected in each library. Two types of cDNAs were iden tified. One type was full-length and contained several disease-resistance g ene (R-gene) signatures. The other type contained several deletions within these signatures. Sequence analyses of the cDNA clones placed them in the T oll-Interleukin-1 receptor, nucleotide binding domain, and leucine-rich rep eat family of disease-resistance genes. Using clone-specific primers from w ithin the 3prime end of the LRRs, we were able to map two cDNA clones (LM6 and MG13) to a BAC contig that is known to span a cluster of disease-resist ance genes.