THE ARYL-HYDROCARBON RECEPTOR INTERACTS WITH TRANSCRIPTION FACTOR IIB

The aryl hydrocarbon receptor (AHR) and its DNA binding partner, the A HR nuclear translocator (ARNT), are basic helix-loop-helix transcripti on factors that mediate many of the toxic and carcinogenic effects of polyhalogenated aromatic hydrocarbons. The basic regions of the AHR an d ARNT contact the GCGTG recognition site, whereas both their helix-lo op-helix domains and periodicity-ARNT-single-minded domains participat e in heterodimerization. To delineate the transcription factors that m ay facilitate DNA binding and transcriptional activation of the AHR/AR NT heterodimer, we questioned whether transcription factor IIB (TFIIB) may interact with either the AHR or ARNT and whether this interaction may affect the ability of the AHR/ARNT complex to bind DNA. Coaffinit y precipitation assays demonstrated that both the AHR and ARNT were ca pable of interacting with TFIIB. Domain mapping experiments revealed t hat TFIIB interacts with the periodicity-ARNT-single-minded and carbox yl-terminal regions of the AHR. To determine whether the interaction b etween TFIIB and the AHR may affect DNA binding of the AHR and ARNT co mplex, we performed gel shift experiments in the absence and presence of TFIIB. The addition of TFIIB significantly increased the formation of the AHR/ARNT DNA binding complex, but only if TFIIB was first allow ed to interact with the AHR before the addition of ARNT. These results indicate that TFIIB interacts with the AHR and may stabilize the DNA binding form of the AHR and thereby augment the ability of the AHR/ARN T complex to interact with its DNA recognition site.