CLONING OF A TELEOST FISH GLUCOCORTICOID RECEPTOR SHOWS THAT IT CONTAINS A DEOXYRIBONUCLEIC ACID-BINDING DOMAIN DIFFERENT FROM THAT OF MAMMALS

In the teleost fish, physiological and biochemical studies suggest tha t glucocorticoids regulate both salt balance and metabolic activities. In mammals, however, these functions are divided between glucocortico ids and mineralocorticoids. In mammals, separate receptors for these t wo classes of steroid hormone have been cloned and sequenced. To begin to understand the regulation in fish of the vital processes ascribed to glucocorticoids, we have cloned, sequenced, expressed, and studied the steroid-binding and transcriptional activation capabilities of the rainbow trout (Onchorhynchus mykiss) glucocorticoid receptor. Norther n blot analysis shows a single rainbow trout GR messenger RNA species of 7.5 kilobases expressed in gill, intestine, skeletal muscle, kidney , and liver. The trout GR 2274-nucleotide coding sequence provides for a protein of 758 amino acids, with appropriate similarities to mammal ian GR, with one striking exception. As in other members of the steroi d/thyroid/retinoid receptor family, the DNA-binding domain contains tw o putative zinc fingers. These have high homology with those of other GRs. However, between the zinc fingers in the trout GR are found 9 mor e amino acids than are seen in mammalian GRs, raising questions as to the functional form of the fish, as opposed to the mammalian, GR. It h as been proposed that as fish appear to use glucocorticoids for both m etabolic and salt control, presumably through a single GR, GR would pr ove to be the evolutionary precursor to mammalian GR and mineralocorti coid receptor (MR). Computer analysis of the known sequences of GRs an d MRs, however, suggests that the fish GR did not give rise to the MR of higher animals, but that both subfamilies of receptor arose from so me earlier gene.