cDNAs encoding two distinct basic helix-loop-helix/ PER-ARNT-SIM (bHLH/PAS) proteins with similarity to the mammalian aryl hydrocarbon nuclear translocator (ARNT) protein were isolated from RTG-2 rainbow trout gonad cells. The deduced proteins, termed rtARNT a and rtARNT b , are identical over the first 533 amino acids and contain a basic helix-loop-helix domain that is 100% identical to human ARNT. rtARNT a and rtARNT b differ in their COOH-terminal domains due to the presence of an additional

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... 373 base pairs of sequence that have the characteristics of an alternatively spliced exon. The presence of the 373-base pair region causes a shift in the reading frame. rtARNT a lacks the sequence and has a COOH-terminal domain of 104 residues rich in proline, serine, and threonine. rtARNT b contains the sequence and has a COOH-terminal domain of 190 residues rich in glutamine and asparagine. mRNAs for both rtARNT splice variants were detected in RTG-2 gonad cells, trout liver, and gonad tissue. rtARNT a and rtARN b protein were identified in cell lysates from RTG-2 cells. Transfection of rtARNT expression vectors into murine Hepa-1 cells that are defective in ARNT function (type II) result in rtARNT protein expression localized to the nucleus. Treatment of these cells with 2,3,7,8-tetrachlorodibenzo-p-dioxin results in a 20-fold greater induction of endogenous P4501A1 protein in cells expressing rtARNT b when compared with rtARNT a , even though both proteins effectively dimerize with the aryl hydrocarbon receptor. The decreased function of rtARNT a appears to be due to inefficient binding of rtARNT a ⅐AHR complexes to DNA. In addition, the presence of rtARNT a can reduce the aryl hydrocarbon receptor-dependent function of rtARNT b in vivo and in vitro.