The glutathione S-transferase D genes. A divergently organized, intronless gene family in Drosophila melanogaster

Abstract

We have characterized a cluster of glutathione S-transferase genes located at 87B on the Drosophila polytene chromosome near the heat shock genes, hsp70. These genes, designated gst Ds in the glutathione S-transferase gene superfamily, are closely linked within a approximately 60-kilobase DNA segment. The gene family has a minimum of eight intronless genes organized in divergent orientations. Two of the genes are probably GST pseudogenes in that their open reading frames are shorter than functional GSTs, and no RNAs from them have been detected thus far. The amino acid sequence identity among the functional genes ranges from 53 to 75% in pairwise comparisons. The intergenic regions are much more AT rich (63-73%) than the coding regions (41-52%), consistent with being promoter/regulatory sequences in Drosophila melanogaster. The mRNA size for each gene suggests that these genes are probably expressed individually from separate promoters. This is the first documentation of definitive physical linkage of a functional glutathione S-transferase multigene family. The genes are divergently organized, and a gradation of sequence similarity exists among the encoded GST isozymes. The patterns of sequence similarity in pairwise comparisons of the family members suggest that gene conversion may have played a role in the evolution of this GST multigene family. We propose that the Drosophila gst D genes provide a unique system for studying GST gene regulation, in vivo physiological functions, and evolution of substrate specificities with a global perspective. The gst D genes in other organisms should be intronless and can be isolated directly from genomic DNAs for functional analyses at the gene and protein levels.