Overexpression of a zebrafish ARNT2-like factor represses CYP1A transcription in ZLE cells

Aryl hydrocarbon receptor nuclear translocator (ARNT) factors belong to a n ovel basic-helix-loop helix-PAS (bHLH-PAS) transcription factor family that controls a variety of physiological and developmental processes. In a prev ious study, we obtained a partial complementary DNA fragment of an ARNT2-li ke factor from zebrafish embryo, liver, and other tissues by reverse transc ription-polymerase chain reaction. In an effort to characterize the functio n of this factor, we screened an embryonic complementary DNA library and ob tained a complete cDNA of this ARNT2-like factor, zARNT2A. The deduced prot ein sequence of zARNT2A encompasses the basic-helix-loop-helix and PAS-A/B motifs and shares highest sequence similarity with the aminoterminal half o f mouse ARNT2 factor. However, it lacks a carboxy-terminal transactivation motif following the PAS-A/B motifs. Transient expression of zARNT2A in cult ured cells resulted in repression of TCDD-dependent CYP1A transcription. Wh ole-mount in situ hybridization revealed that zARNT2A is expressed in brain and pronephros at prime-5 stages. In adult fish, zARNT2A messenger RNA is transcribed in a wide range of tissues, which indicates that zARNT2A and it s corresponding signal transduction mechanisms have important roles in fish development and other physiological aspects.