FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

Abstract

hreonine synthase (TS) and dihydrodipicolinate synthase (DHDPS) are enzymes of the aspartate family pathway leading to biosynthesis of Thr and Lys in plants and microorganisms, respectively. Sequence analysis of the cDNA of those enzymes from rice revealed that they harbor a full-length open reading frame for OsTS encoding for 521 amino acids, corresponding to a protein of approximately 57.2 kDa and OSDHDPS also encoding for 380 amino acids, corresponding to a protein of approximately 41.4 kDa. The predicted amino acid sequence of OsTS and OsDHDPS are highly homologous to those of Arabidopsis and many bacterial respective sequences that are encoded for thrC and dapA gene, respectively. The OsTS protein harbors a signature binding motif for pyridoxal-5’- phosphate at the amino terminus. The Os7S expression was complemented by a thrC mutant strain of E. coli. The OsTS expression was correlated with the survival of the thrC mutant, which is affected by the supplementation of an Asp pathway metabolite, Met. Expression of OsDHDPS in dapA mutants of E. coli showed that the gene was able to functionally complement with the mutant. The result suggested that the OsTS and OsDHDPS encode a protein TS and DHDPS, respectively in rice.