Operator derepressed mutations in the proline utilisation gene cluster of Aspergillus nidulans.

The proline utilisation gene cluster of Aspergillus nidulans can be repressed efficiently only when both repressing nitrogen and repressing carbon sources are present. We show that two cis-acting mutations in this cluster permit the efficient transcription of the prnB gene under repressing conditions, resulting in direct or indirect derepression of two other transcripts of the pathway. These mutations are transitions that define a 5'GAGACCCC3' sequence. Similar sequences are found upstream of other genes subject to carbon catabolite repression. We propose that this sequence defines the binding site for the negatively-acting CreA protein, which mediates carbon catabolite repression in this fungus.