Neuronal PAS domain protein 1 (NPAS1), a basic helix-loophelix-PAS transcription factor expressed in the central nervous system, has been suggested to be involved in neuronal differentiation. However, relatively little is known about the molecular mechanism underlying the role of NPAS1 during development. In this study we set out to characterize the different domains within NPAS1. We showed that the nuclear localization of NPAS1 is dependent on the presence of ARNT. In addition, the

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... nal potential of ARNT is not required for this localization. In the absence of ARNT, NPAS1 is excluded from the nucleus, and this exclusion is due to the presence of a nuclear export signal within the N terminus of NPAS1. The interaction between NPAS1 and ARNT is via their N termini. We found no transactivation domain within NPAS1; instead, we mapped out at least three repression domains within NPAS1, suggesting that NPAS1 acts as a repressor. Furthermore, our experiments showed that NPAS1 is able to repress the transactivation functions of ARNT and ARNT2. We suggest that NPAS1 is guided into the nucleus by ARNT via the ARNT nuclear localization signal, and NPAS1 can override the activation function of adjacent transcription factors, providing a mechanism by which NPAS1 may inhibit transcription.