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Molecular and Cellular Biology, February 2007, p. 791-802, Vol. 27, No. 3
0270-7306/07/$08.00+0     doi:10.1128/MCB.00761-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Nuclear Export of the Transcription Factor NirA Is a Regulatory Checkpoint for Nitrate Induction in Aspergillus nidulans ^,

Andreas Bernreiter,¹ Ana Ramon,^2, Javier Fernández-Martínez,³ Harald Berger,¹ Lidia Araújo-Bazan,³ Eduardo A. Espeso,³ Robert Pachlinger,¹ Andreas Gallmetzer,¹ Ingund Anderl,^1, Claudio Scazzocchio,^2,4 and Joseph Strauss^1*

Fungal Genetics and Genomics Unit, Austrian Research Centers and BOKU Vienna, Muthgasse 18, A-1190 Vienna, Austria,¹ Institut de Genetique et Microbiologie, Université Paris-Sud, F-91405 Orsay Cedex, France,² Centro de Investigaciones Biológicas, CSIC, Madrid 28040, Spain,³ Institut Universitaire de France, Paris, France⁴

Received 16 March 2006/ Returned for modification 31 July 2006/ Accepted 30 October 2006

NirA, the specific transcription factor of the nitrate assimilation pathway of Aspergillus nidulans, accumulates in the nucleus upon induction by nitrate. NirA interacts with the nuclear export factor KapK, which bridges an interaction with a protein of the nucleoporin-like family (NplA). Nitrate induction disrupts the NirA-KapK interaction in vivo, whereas KapK associates with NirA when this protein is exported from the nucleus. A KpaK leptomycin-sensitive mutation leads to inducer-independent NirA nuclear accumulation in the presence of the drug. However, this does not lead to constitutive expression of the genes controlled by NirA. A nirA^c1 mutation leads to constitutive nuclear localization and activity, remodeling of chromatin, and in vivo binding to a NirA upstream activation sequence. The nirA^c1 mutation maps in the nuclear export signal (NES) of the NirA protein. The NirA-KapK interaction is nearly abolished in NirA^c1 and NirA proteins mutated in canonical leucine residues in the NirA NES. The latter do not result in constitutively active NirA protein, which implies that nuclear retention is necessary but not sufficient for NirA activity. The results are consistent with a model in which activation of NirA by nitrate disrupts the interaction of NirA with the NplA/KapK nuclear export complex, thus resulting in nuclear retention, leading to AreA-facilitated DNA binding of the NirA protein and subsequent chromatin remodeling and transcriptional activation.

Published ahead of print on 20 November 2006.

Supplemental material for this article may be found at http://mcb.asm.org/.

Present address: Sección Bioquímica, Departamento de Biología Celular y Molecular, Facultad de Ciencias, Universidad de la República Iguá, 4225, CP 11400 Montevideo, Uruguay.

Present address: Institute for Molecular Biotechnology, Technical University Graz, A-8020 Graz, Austria.

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