Cloning of a rice tau class GST isozyme and characterization of its substrate specificity

The GST cDNA was successfully cloned from an Oryza sativa cDNA library by PCR using oligonucleotide primers based on the OsGSTU5 (GenBank Accession No. AF309377) sequence. The cDNA was composed of a 687-bp open reading frame encoding for 228 amino acids. The deduced amino acid sequence of this gene shared over 60% sequence identity with the sequences of the tau class ZmGSTU6 and ZmGSTU19. This gene was expressed in Escherichia coli with the pET vector system and the gene product was purified to homogeneity by GSH-Sepharose affinity column chromatography. The expressed OsGSTU5 formed a homo-dimer composed of 25 kDa subunit and its pI value was approximately 7.5. The OsGSTU5 displays high activities towards 1-chloro-2,4-dinitrobenzene and 1,2-epoxy-3-(p-nitrophenoxy)propane. The activity of the OsGSTU5 was significantly inhibited by hematin and ethacrynic acid. The OsGSTU5 shows the highest activity towards chloro-s-triazine and acetanilide herbicides.