Distinctive structure of the human GSTM3 gene-inverted orientation relative to the mu class glutathione transferase gene cluster.

The sequence and exon-intron structure of the human class mu GSTM3 glutathione transferase gene and its orientation with respect to the remainder of the human class mu GSTM gene cluster were determined. The GSTM3 gene 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, the GSTM3 gene does not share significant sequence similarity with other class mu glutathione transferase genes. Identification of overlapping cosmid clones that span the region between GSTM5, the next nearest glutathione transferase gene, and GSTM3 showed that the two genes are about 20,000 bp apart. PCR primers developed from sequences 3'-downstream from the GSTM5 gene were used to identify clones containing the GSTM3 gene. Amplification with these primers showed that the orientation of the GSTM3 gene 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.