On how a transcription factor can avoid its proteolytic activation in the absence of signal transduction

In response to alkaline ambient pH, the Aspergillus nidulans PacC transcrip tion factor mediating pH regulation of gene expression is activated by prot eolytic removal of a negative-acting C-terminal domain. We demonstrate inte ractions involving the similar to 150 C-terminal PacC residues and two regi ons located immediately downstream of the DNA binding domain. Our data indi cate two full-length PacC conformations whose relative amounts depend upon ambient pH: one 'open' and accessible for processing, the other 'closed' an d inaccessible. The location of essential determinants for proteolytic proc essing within the two more upstream interacting regions probably explains w hy the interactions prevent processing, whereas the direct involvement of t he C-terminal region in processing-preventing interactions explains why C-t erminal truncating mutations result in alkalinity mimicry and pH-independen t processing. A mutant PacC deficient in pH signal response and consequent processing behaves as though locked in the 'closed' form. Single-residue su bstitutions, obtained as mutations bypassing the need for pH signal transdu ction, identify crucial residues in each of the three interactive regions a nd overcome the processing deficiency in the 'permanently closed' mutant.