Gene expression of rat and human microsomal glutathione S-transferases

We have isolated and characterized cDNA clones for both the rat and human liver microsomal GSH S-transferase (mGST) mRNAs. The rat sequence is 883 nucleotides long with 5'- and 3'-untranslated regions of 55 and 360 nucleotides, respectively, and contains two polyadenylation signals. The highest level of mRNA expression occurs in the rat liver, with widely varied levels of expression in extrahepatic tissues. Primer extension of rat liver poly(A) RNAs showed that only 12 nucleotides exist beyond the isolated cDNA. Southern blotting analysis of rat genomic DNA showed that this gene resides in a single EcoRV band of approximately 23 (kilobases) kb. Digestions with other restriction enzymes were consistent with the microsomal GST gene being a single copy gene with at least 3 exons spanning less than 12 kb. Human microsomal GST cDNAs were also characterized, and a 909 nucleotide sequence was determined with 5'- and 3'-untranslated regions of 74 and 368 nucleotides, respectively. RNA blot hybridization of human liver mRNA demonstrated that the human mGST is very similar in size to the rat mGST poly(A) RNA. Both the rat and human sequences contain a 154-amino acid reading frame and both show the same seven amino acid changes compared to the previously published protein sequence which upon reevaluation reveals the cDNA-deduced sequence to be correct. These two transcripts share 77% nucleotide similarity in the coding region, 47% in the 5'-untranslated regions and 61% in the 3'-untranslated regions. The rat and human microsomal GSTs show 95% conservation in amino acids. Several lines of evidence suggest that neither transcript contains a membrane-directing cleavable signal sequence.