CHARACTERIZATION OF AN ARABIDOPSIS CDNA FOR A SOLUBLE EPOXIDE HYDROLASE GENE THAT IS INDUCIBLE BY AUXIN AND WATER-STRESS

A cDNA (1122 bp) was isolated from a cDNA library prepared from Arabid opsis thaliana L. that had been subjected to drought stress for 1 h. T he sequencing of a genomic clone corresponding to the cDNA and S1 mapp ing analysis revealed that the cDNA lacked the first 6 bp from its tra nslational start (ATG). The resulting open reading frame encodes a pol ypeptide of 321 amino acids, and the calculated molecular weight of th is polypeptide is 36 423 Da. The deduced amino acid sequence shows a h igh degree of similarity to C terminal halves of those of soluble epox ide hydrolases (sEHs) of human, mouse and rat, 35.5%, 34.1% and 33.1%, respectively. The cDNA was expressed in Escherichia coli cells, and t he expressed protein migrates at 40 kDa when analyzed by SDS-PAGE. The recombinant protein at 40 kDa is much smaller than the mammalian sEH (58 kDa) but has characteristics of activity and inhibition similar to the mammalian sEHs when assayed with the substrate trans-stilbene oxi de and the inhibitors 4-fluorochalcone oxide (4FCO), (2R,3R)-3-(4-nitr ophenyl) glycidol (RRNPG), and (2S,3S)-3-(4-nitrophenyl)glycidol (SSNP G), which indicates that the cDNA did encode a soluble epoxide hydrola se of A. thaliana (AtsEH). Drought stress, but not heat or cold stress , slightly increased the accumulation of the mRNAs for AtsEH. The leve l of AtsEH transcripts increased strongly after treatment with a plant hormone, auxin (2,4-dichlorophenoxyacetic acid, 2,4-D; naphthaleneace tic acid, NAA; and indole-3-acetic acid, IAA) in young, pre-bolting pl ants. Treatment with cytokinin (6-benzylaminopurine, BA), abscisic aci d (ABA) or gibberellin (GA(3)) had no detectable effect on AtsEH trans cript levels. The transcripts for AtsEH gene were detected in the aeri al vegetative organs of bolting plants (i.e. stems and leaves), but no t in roots, flowers and seeds. The possible function of AtsEH is discu ssed. A similar sEH cDNA has recently been characterized in potato (St apleton et al., 1994).