IDENTIFICATION OF A CELL-SPECIFIC TRANSCRIPTION ACTIVATION DOMAIN WITHIN THE HUMAN AH RECEPTOR NUCLEAR TRANSLOCATOR

In the presence of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and rela ted chemicals, the Ah receptor nuclear translocator (Arnt) forms a het erodimeric complex with the ligand-bound Ah receptor, leading to recog nition of dioxin-responsive elements within the enhancer of the CYP1A1 gene and transcription activation by an unknown mechanism. To underst and the role of Arnt in transcription activation by the Ah receptor-Ar nt heterodimer, we performed a deletion analysis of Arnt to locate dom ains that are directly involved in transcription activation. We showed that the C-terminal 34 amino acids of Arnt encode a transcription act ivation domain (TAD) that functions independently of other sequences i n the Ah receptor complex when attached to the heterologous Gal4 DNA b inding domain. Deletion of the C-terminal acidic-rich 14 amino acids c ompletely abolishes activity. Sequences important in Arnt TAD function were independent of the glutamine-rich region which is an important s tructural feature in the TAD of other transcription factors. The stren gth of the Arnt TAD when compared with the strong TAD from the herpes simplex virus VP16 protein was cell-type specific. Both the Arnt and V P16 TAD were equally strong in COS-1 cells, but the Arnt TAD had weak activity in an Arnt-deficient mouse hepatoma cell line and was not nee ded for restoration of CYP1A1 activation. These results imply that for CYP1A1 activation the Ah receptor provides the dominant activation fu nction for the heterodimer in hepatoma cells. The potential of the Arn t TAD to contribute to activation by the Ah receptor complex is likely determined by availability or activity of cell-specific factors with which the TAD interacts. (C) 1996 Academic Press, Inc.