Conserved domains e.g. nucleotide binding site (NBS) were found in several cloned plant disease resistance genes. Based on the NBS domain, resistance gene analogs (RGAs) have been isolated previously and were used as probes to screen a soybean (Glycine max L. Merr.) cDNA library. A full-length cDNA, KR3, was obtained by screening the library and rapid amplification of cDNA ends (RACE) method. Sequence analysis revealed that the cDNA is 2 353 bp in length and the open reading frame (ORF) codes for a polypeptide of 636 amino acids with a Toll-Interleukin-1 receptor (TIR) and a NBS domain. Sequence alignment showed that it was similar to N gene of tobacco. The phylogenetic tree analysis of R proteins with NBS from higher plants was performed. The KR3 gene has low copies in soybean genome and its expression was induced by exogenous salicylic acid (SA).
Previously the partial sequence of an ethylene receptor gene NTHK2 was isolated from tobacco (Nicotiana tabacum L. var. Xanthi) plants and it was wound and drought inducible. In the present study full-length cDNA of NTHK2 was cloned by 5'-RACE method. NTHK2 gene has 3 216 bp, with 509 bp of 5'-non-coding region and 427 lip of 3'-non-coding region, and encodes an ethylene-receptor homolog of 760, amino acids. NTHK2 protein has a putative signal peptide, three transmembrane domains, a histidine kinase domain and a receiver domain. In the putative histidine kinase domain, the histidine at the phosphorylation site was replaced by an asparagine. To study the biochemical property of NTHK2, its kinase domain was expressed as a fusion protein with glutathione S-transferase (GST) using yeast Schizzosaccharomyces pombe as an expression system. In vitro kinase assay showed that NTHK2 kinase domain can autophosphorylate in the presence of Mg2+, indicating that NTHK2 may function as a kinase. Further studies will elucidate the function of NTHK2 in plant.
