Detection of Ah receptor and Ah receptor nuclear translocator mRNAs in the oocytes and developing embryos of zebrafish (Danio rerio)

The heterodimeric complex of Ah receptor (AHR) - Ah receptor nuclear translocator (ARNT) is an ubiquitous transcription factor which mediates the expression of vertebrate xenobiotic-response genes, such as CYP1A1 and CYP1A2. AHR also performs key functions in murine tissue differentiation. Both AHR and ARNT factors share conserved function domains with PAS domain families, such as Drosophila CNS-developing modulator, SIM, and biological rhythm factor, PER. By using RT-PCR technique, we have obtained partial cDNA fragments of zebrafish AHR and ARNT from fish tissues. We found that both ahr and arnt genes are active during fish embryogenesis. The mRNAs of AHR and ARNT are also transcribed in oocytes as maternal mRNA. The deduced amino acid sequences derived from the amplified cDNA fragments share significant homology with the respective mammalian AHR and ARNT PAS domain sequences. The xenobiotic, 2,3,7,8-TCDD, strongly induces zebrafish CYP1A expression during embryogenesis. Apparently, both AHR and ARNT factors are present in fish embryos in forms that can accurately respond to the proper ligands to induce CYP1A. We speculate these factors play similar functional roles in fish development as they do during development of murine embryos.