THE REGULATOR OF NITRATE ASSIMILATION IN ASCOMYCETES IS A DIMER WHICHBINDS A NONREPEATED, ASYMMETRICAL SEQUENCE

The regulation of nitrate assimilation seems to follow the same patter n in all ascomycetes where this process has been studied. We show here by in vitro binding studies and a number of protection and interferen ce techniques that the transcription factor mediating nitrate inductio n in Aspergillus nidulans, a protein containing a binuclear zinc clust er DNA binding domain, recognizes an asymmetrical sequence of the form CTCC GHGG. We further show that the protein binds to its consensus si te as a dimer. We establish the role of the putative dimerization elem ent by its ability to replace the analogous element of the cI protein of phage lambda. Mutagenesis of crucial leucines of the dimerization e lement affect both the binding ability of the dimer and the conformati on of the resulting protein-DNA complex. This is the first case to be described where a dimer recognizes such an asymmetrical nonrepeated se quence, presumably by each monomeric subunit making different contacts with different DNA half-sites.