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Mol Cell Biol, March 1998, p. 1339-1348, Vol. 18, No. 3
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

The Regulator of Nitrate Assimilation in Ascomycetes Is a Dimer Which Binds a Nonrepeated, Asymmetrical Sequence

Joseph Strauss,1,2,dagger M. Isabel Muro-Pastor,1 and Claudio Scazzocchio1,*

Institut de Génétique et Microbiologie, Université Paris-Sud, URA D2225, 91405 Orsay Cedex, France,1 and Institut für Biochemische Technologie und Mikrobiologie, Technische Universität Wien Vienna, Austria2

Received 6 October 1997/Returned for modification 13 November 1997/Accepted 11 December 1997

The regulation of nitrate assimilation seems to follow the same pattern in all ascomycetes where this process has been studied. We show here by in vitro binding studies and a number of protection and interference techniques that the transcription factor mediating nitrate induction in Aspergillus nidulans, a protein containing a binuclear zinc cluster DNA binding domain, recognizes an asymmetrical sequence of the form CTCCGHGG. We further show that the protein binds to its consensus site as a dimer. We establish the role of the putative dimerization element by its ability to replace the analogous element of the cI protein of phage lambda . Mutagenesis of crucial leucines of the dimerization element affect both the binding ability of the dimer and the conformation of the resulting protein-DNA complex. This is the first case to be described where a dimer recognizes such an asymmetrical nonrepeated sequence, presumably by each monomeric subunit making different contacts with different DNA half-sites.

* Corresponding author. Mailing address: Institut de Génétique et Microbiologie, Université Paris-Sud, URA D2225, 91405 Orsay Cedex, France. Phone: 33 1 69156356. Fax: 33 1 69157808. E-mail: scazzocchio{at}igmors.u-psud.fr.

dagger Present address: Institut für Biochemische Technologie und Mikrobiologie, Technische Universität Wien, Vienna, Austria.




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