MOLECULAR EVOLUTION OF 2 VERTEBRATE ARYL-HYDROCARBON (DIOXIN) RECEPTORS (AHR1 AND AHR2) AND THE PAS FAMILY

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcriptio n factor through which halogenated aromatic hydrocarbons such as 2,3,7 ,8-tetrachloro-dibenzo-p-dioxin (TCDD) cause altered gene expression a nd toxicity. The AHR belongs to the basic helix-loop-helix/Per-ARNT-Si m (bHLH-PAS) family of transcriptional regulatory proteins, whose memb ers play key roles in development, circadian rhythmicity, and environm ental homeostasis; however, the normal cellular function of the AHR is not yet known. As part of a phylogenetic approach to understanding th e function and evolutionary origin of the AHR, we sequenced the PAS ho mology domain of AHRs from several species of early vertebrates and pe rformed phylogenetic analyses of these AHR amino acid sequences in rel ation to mammalian AHRs and 24 other members of the PAS family. AHR se quences were identified in a teleost (the killifish Fundulus heterocli tus), two elasmobranch species (the skate Raja erinacea and the dogfis h Mustelus canis), and a jawless fish (the lamprey Petromyzon marinus) . Two putative AHR genes, designated AHR1 and AHR2, were found both in Fundulus and Mustelus. Phylogenetic analyses indicate that the AHR2 g enes in these two species are orthologous, suggesting that an AHR gene duplication occurred early in vertebrate evolution and that multiple AHR genes may be present in other vertebrates. Database searches and p hylogenetic analyses identified four putative PAS proteins in the nema tode Caenorhabditis elegans, including possible AHR and ARNT homologs. Phylogenetic analysis of the PAS gene family reveals distinct clades containing both invertebrate and vertebrate PAS family members; the la tter include paralogous sequences that we propose have arisen by gene duplication early in vertebrate evolution. Overall, our analyses indic ate that the AHR is a phylogenetically ancient protein present in all living vertebrate groups (with a possible invertebrate homolog), thus providing an evolutionary perspective to the study of dioxin toxicity and AHR function.