Ligand binding by the aryl hydrocarbon receptor (AhR), a member of the bHLH-PAS family of transcriptional reg-ulatory proteins, has been mapped to a region within the second ÔPASÕ domain, a conserved sequence motif ®rst discovered in the Per-ARNT-Sim family of proteins. In addition to the bacterial photoactive yellow protein (PYP), which had been proposed as a structural prototype for the three dimensional fold of PAS domains, two crystal structures of the PAS domain have recently been deter-mined: the human potassium channel HERG and the heme binding domain of the bacterial O2 sensing FixL protein. .
