The aryl hydrocarbon receptor interacts with transcription factor IIB

The aryl hydrocarbon receptor (AHR) and its DNA binding partner, the AHR nuclear translocator (ARNT), are basic helix-loop-helix transcription factors that mediate many of the toxic and carcinogenic effects of polyhalogenated aromatic hydrocarbons. The basic regions of the AHR and ARNT contact the GCGTG recognition site, whereas both their helix-loop-helix domains and periodicity-ARNT-single-minded domains participate in heterodimerization. To delineate the transcription factors that may facilitate DNA binding and transcriptional activation of the AHR/ARNT 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. Coaffinity precipitation assays demonstrated that both the AHR and ARNT were capable of interacting with TFIIB. Domain mapping experiments revealed that TFIIB interacts with the periodicity-ARNT-single-minded and carboxyl-terminal regions of the AHR. To determine whether the interaction between TFIIB and the AHR may affect DNA binding of the AHR and ARNT complex, 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 allowed 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/ARNT complex to interact with its DNA recognition site.