The structure of two murine class-mu glutathione transferase genes coordinately induced by butylated hydroxyanisole

Abstract

We report the sequences of two coordinately induced murine glutathione transferase genes, mGSTM1 (GT8.7, Yb1) and mGSTM3 (GT9.3). Genomic clones covering the entire mGSTM1 gene were isolated; comparison of the mGSTM1 gene with genomic sequences from rat class-mu glutathione transferase genes suggests that the mGSTM1 gene is orthologous to the rGSTM1 (rat3, Yb1) gene. The start of mGSTM1 mRNA transcription was mapped by primer extension and RNase protection to 37 nucleotides upstream from the initiation codon. The 160 nucleotides 5'-proximal to the start of transcription match exactly the 5'-end of the class-mu glutathione transferase cDNA clone pmGT10. An mRNA transcript was found approximately 2.0 kb upstream from the start of mGSTM1 transcription; its sequence does not show significant similarity to other sequences in the DNA or protein sequence databases. The mGSTM1 gene contains a TATAAA sequence at -31 nucleotides upstream from the start of transcription, but no exact match to the antioxidant response element (RGTGACNNNGC), the xenobiotic response element (TNGCGTG), or the AP-1 consensus (TGASTMA) is found in the 5'-flanking region, although near matches are found in the 5'-flanking region, in intron 1, and in other parts of the gene. A genomic clone containing the first five exons of the mGSTM3 gene was also isolated. The mGSTM3 gene contains several repetitive elements--two upstream from the start of transcription and one within intron 2--that disrupt its similarity with the mGSTM1 gene. The 5'-flanking sequence of the mGSTM3 gene does not contain a TATAAA sequence or any exact matches to ARE or XRE consensus sequences, although an Sp1 binding site is found at -66. mGSTM3 and mGSTM1 diverge substantially outside their exons and share less than 60% sequence identity in the 5'-flanking region. Thus, it is likely that the mGSTM1-mGSTM3 gene duplication predates the rat-mouse divergence. The strongest region of conservation between the mGSTM1 gene and the mGSTM3 gene occurs in exon 3, intron 3, and exon 4; this region also shares strong similarity with the rGSTM1 (rat3, Yb1) and rGSTM2 (rat4, Yb2) genes.