Three repeated glutathione S-transferase genes from a marine fish, the plaice (Pleuronectes platessa)

Abstract

The cytosolic glutathione S-transferases (GSTs) catalyse the transfer of glutathione to a variety of xenobiotic and toxic endogenous compounds. This results in detoxification of the offending chemical, and the resulting conjugate is able to enter the organism's excretion pathways. The major GST of plaice (Pleuronectes platessa) liver, GSTA, is structurally related to mammalian theta class GSTs and also to GSTs from plants and insects. GST genes are known to be induced in animals and plants by a wide range of xenobiotic chemicals and by oxidative stress, and our interest is in the regulation of GST genes from plaice. Screening of a plaice genomic DNA library with GSTA cDNA resulted in the isolation of two overlapping clones. Analysis of these clones revealed the presence of the gene for GSTA, designated GSTA, and also two more putative genes for closely related GSTs, designated GSTA1 and GSTA2. The exon structures of the three GST genes are very similar and the predicted amino acid sequences show 60-70% homology. Promoter analysis of the regions upstream of GSTA and GSTA1 were shown to have activity in a turbot fibroblast cell line, but the region upstream of GSTA2 was inactive in this system. The promoter active regions of GSTA contain sequence elements which have been shown to respond to oxidative stress in mammals, and the regions upstream of GSTA1 contain oestrogen and peroxisomal proliferator response elements. Thus we have shown that these two closely related genes are physically close together in the plaice genome but we believe them to be under separate control and to respond to different signals and stressors.