Refining the pH response in Aspergillus nidulans: a modulatory triad involving PacX, a novel zinc binuclear cluster protein.

The Aspergillus nidulans PacC transcription factor mediates gene regulation in response to alkaline ambient pH which, signalled by the Pal pathway, results in the processing of PacC(72) to PacC(27) via PacC(53). Here we investigate two levels at which the pH regulatory system is transcriptionally moderated by pH and identify and characterise a new component of the pH regulatory machinery, PacX. Transcript level analysis and overexpression studies demonstrate that repression of acid-expressed palF, specifying the Pal pathway arrestin, probably by PacC(27) and/or PacC(53), prevents an escalating alkaline pH response. Transcript analyses using a reporter and constitutively expressed pacC trans-alleles show that pacC preferential alkaline-expression results from derepression by depletion of the acid-prevalent PacC(72) form. We additionally show that pacC repression requires PacX. pacX mutations suppress PacC processing recalcitrant mutations, in part, through derepressed PacC levels resulting in traces of PacC(27) formed by pH-independent proteolysis. pacX was cloned by impala transposon mutagenesis. PacX, with homologues within the Leotiomyceta, has an unusual structure with an amino-terminal coiled-coil and a carboxy-terminal zinc binuclear cluster. pacX mutations indicate the importance of these regions. One mutation, an unprecedented finding in A. nidulans genetics, resulted from an insertion of an endogenous Fot1-like transposon.