Functional analysis of a gene encoding threonine synthase from rice

Threonine synthase (TS) is a pyridoxal phosphate dependent enzyme that catalyzes the formation of threonine (Thr) through O-phosphohomoserine (OPH) from the aspartate family pathway in plants. The properties of the TS enzyme have been evaluated in many bacteria and few plants. Sequence analysis of the cDNA from rice revealed that it harbors a full-length open reading frame for OsTS encoding for 521 amino acids, corresponding to a protein of approximately 57.2 kD. The predicted amino acid sequence of OsTS is highly homologous to that of Arabidopsis TS and many bacterial TS encoded by thrC gene. The OsTS protein harbors a signature binding motif for pyridoxal- 5’ -phosphate at the amino terminus. A thrC mutant strain of Escherichia coli was complemented by OsTS expression. OsTS expression was correlated with the survival of the thrC mutant, which is affected by the supplementation of an aspartate pathway metabolite, methionine.