Distinctive Structure of the HumanGSTM3Gene—Inverted Orientation Relative to the Mu Class Glutathione Transferase Gene Cluster

The sequence and exon–intron structure of the human class muGSTM3glutathione transferase gene and its orientation with respect to the remainder of the human class muGSTMgene cluster were determined. TheGSTM3gene is 2847 bp long and is thus considerably shorter than the other class mu genes in the cluster, which range in size from 5325 to 7212 bp. Outside the protein-coding region, theGSTM3gene does not share significant sequence similarity with other class mu glutathione transferase genes. Identification of overlapping cosmid clones that span the region betweenGSTM5,the next nearest glutathione transferase gene, andGSTM3showed that the two genes are about 20,000 bp apart. PCR primers developed from sequences 3′-downstream from theGSTM5gene were used to identify clones containing theGSTM3gene. Amplification with these primers showed that the orientation of theGSTM3gene is 5′-GSTM5-3′–3′-GSTM3-5′. Long-range PCR reactions confirmed this orientation both in the GSTM-YAC2 YAC clone, which contains the five class mu glutathione transferase genes on chromosome 1, and in human DNA. This tail-to-tail orientation is consistent with an evolutionary model of class mu glutathione transferase divergence from a pair of tail-to-tail “M1-like” and “M3-like” class mu glutathione transferase genes that was present at the mammalian radiation to the current organization of multiple head-to-tail M1-like genes tail-to-tail with a single M3-like gene with distinct structural properties and expression patterns.