ROLE OF THE PAS DOMAIN IN REGULATION OF DIMERIZATION AND DNA-BINDING SPECIFICITY OF THE DIOXIN RECEPTOR

The dioxin receptor is a ligand-regulated transcription factor that me diates signal transduction by dioxin and related environmental polluta nts. The receptor belongs to the basic helix-loop-helix (bHLH)-Per-Arn t-Sim (PAS) family of factors, which, in addition to the bHLH motif, c ontain a PAS region of homology. Upon activation, the dioxin receptor dimerizes with the bHLH-PAS factor Arnt, enabling the receptor to reco gnize xenobiotic response elements in the vicinity of target genes. We have studied the role of the PAS domain in dimerization and DNA bindi ng specificity of the dioxin receptor and Arnt by monitoring the abili ties of the individual bHLH domains and different bHLH-PAS fragments t o dimerize and bind DNA in vitro and recognize target genes in vivo. T he minimal bHLH domain of the dioxin receptor formed homodimeric compl exes, heterodimerized with full-length Amt, and together with Arnt was sufficient for recognition of target DNA in vitro and in vivo. In a s imilar fashion, only the bHLH domain of Amt was necessary for DNA bind ing specificity in the presence of the dioxin receptor bHLH domain. Mo reover, the bHLH domain of the dioxin receptor displayed a broad dimer ization potential, as manifested by complex formation with, e.g., the unrelated bHLH-Zip transcription factor USP. In contrast, a construct spanning the dioxin receptor bHLH domain and an N-terminal portion of the PAS domain failed to form homodimers and was capable of dimerizing only with Amt. Thus, the PAS domain is essential to confer dimerizati on specificity of the dioxin receptor.