Genome-wide analysis of the Tetrahymena thermophila glutathione S-transferase gene superfamily

The ciliate Tetrahymena thermophila has a rapid response to detoxify xenobiotics, which presents opportunity to study the diversification of Glutathione S-Transferase superfamily. In-silico identification of putative GST genes were resulted with 70 GST genes; 49 TtGSTmu, 7 TtGSTomega, 5 TtGSTtheta, 2 TtGSTzeta, 4 TtMAPEG and 3 TtEF1G. TtGST superfamily has short intron carrying or intronless genes. The most expressed mRNAs of TtGST are limited to 4 members at all life stages. TtGST genes are widely distributed to all five micronuclear chromosomes with the highest diversified members from different classes in chromosome 4. The clustering and the orientation of some TtGSTs in the T. thermophila genome give clues about the recent gene duplication. Analysis of GSH affinity-purified GST proteins with Western blot and activity assay showed GST activity carrying purified TtGST populations. In conclusion, the enhanced genome capacity of TtGST superfamily may have evolved through improved GST enzymatic activity.     

A second stress-inducible glutathione S-transferase gene from Schizosaccharomyces pombe

A second glutathione S-transferase gene (GST II) was isolated from the chromosomal DNA of the fission yeast Schizosaccharomyces pombe. The nucleotide sequence determined contains 1908 bp including an open reading frame of 230 amino acids that would encode a protein of a molecular mass of 26843.4 Da. The amino acid sequence of the putative GST II is very homologous with that of the previously isolated GST gene (GST I) located in the same chromosome III of S. pombe. The cloned GST II gene produces the functional GST in S. pombe, and it gives much higher GST in the stationary phase than in the exponential phase. Regulation of the GST II gene was studied using the GST II-lacZ fusion. The synthesis of beta-galactosidase from the fusion plasmid is greatly enhanced by the treatments with oxidative stresses such as menadione and mercuric chloride. It is also induced by o-dinitrobenzene, one of the GST substrates. NO-generating S-nitroso-N-acetylpenicillamine has a weak induction effect on the expression of GST II gene. These results indicate that the S. pombe GST II gene is involved in the oxidative stress response and detoxification. However, physiological meaning on the existence of the two similar GST genes in S. pombe remains unknown yet.