Worm Breeder's Gazette 14(5): 72 (February 1, 1997)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

A C. elegans Dioxin Receptor

Jo Anne Powell-Coffman, Bill Wood

Molecular, Cellular and Developmental Biology, University of Colorado, Boulder C0 80309-0347

        The vertebrate aryl hydrocarbon receptor (AHR; a.k.a. the dioxin
receptor) mediates the teratogenic and carcinogenic effects of certain
environmental contaminants.  AHR ligands include benzo(a)pyrene, the
primary carcinogen in cigarette smoke, and
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a ubiquitous man-made
pollutant.  Unliganded AHR resides in the cytoplasm in a complex with
the 90-kDa heat shock protein (HSP90).  Upon binding ligand, AHR
translocates to the nucleus, dissociates from HSP90, and dimerizes with
the AHR nuclear translocator protein (ARNT).  Both AHR and ARNT belong
to a class of regulatory proteins that contain a basic-helix-loop-helix
DNA binding motif and a PAS domain (also found in Drosophila period and
single-minded).  The AHR-ARNT heterodimer binds specific promoter
sequences to regulate transcription. (1)  In mice AHR function is
required for the proper development of the immune system and liver, but
the presumptive endogenous ligand(s) is unknown. (2)  Until recently, it
had not been possible to study AHR signaling in a model system amenable
to genetic analysis, as no invertebrate orthologues of AHR or ARNT had
been reported.

        We have cloned the C. elegans homologues of AHR and ARNT.  CeAHR
may be the egl-33 gene; it is included in the 15kb genomic fragment that
rescues egl-33 (ct315). (3)  CeARNT maps to the right of egl-33 on LGI. 
We have expressed these gene products in rabbit reticulocyte lysates. 
Initial gel electrophoretic shift experiments demonstrate that CeAHR and
CeARNT interact to bind the mammalian AHR-ARNT enhancer site. 
Competition experiments with wild-type and a mutant form of the enhancer
site confirm that this binding is specific.  These data suggest that we
have cloned true C. elegans orthologues of AHR and ARNT.  We are also
collaborating with C. Bradfield and colleagues at the University of
Wisconsin to determine the binding affinity of CeAHR for TCDD.

        To identify the cellular decisions regulated by the AHR
signaling complex in C. elegans, we are screening for Tc1-induced
deletions in CeAHR and CeARNT, and we are examining the expression
patterns of these two genes.  RNA blots indicate that CeAHR and CeARNT
are coordinately expressed and are most abundant during embryogenesis
and the first larval stage.  We have constructed lacZ and GFP reporter
genes and have detected expression of CeAHR::lacZ as early as the
30-cell stage.  At the 300-cell stage, it is expressed in approximately
20% of the cells.  As morphogenesis begins, most cells cease to express
CeAHR::lacZ, and at hatching, it is detectable in a subset of neuronal
and hypodermal cells.  These experiments will establish the foundation
for further genetic analysis of the AHR signaling pathway.

1.  Hankinson, O.  (1995)  Annu. Rev. Pharmacol. Toxicol. 35: 307-34;
     Whitlock, J. P. (1993)  Chem. Res. Toxicol. 6: 754-763; Bock, K. W.
     (1993)  Rev. Phys. Biochem. Pharmacol. 125: 1-42

2.  Fernandez-Salguero, P. et al.  (1995)  Science 268: 722-726

3.  Powell-Coffman, J. A. and B. Wood.  1996 West Coast Worm Mtg.