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

Summary The A spergillus nidulans PacC transcription factor mediates gene regulation in response to alkaline ambient pH which, signalled by the P al 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 pal F , 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 pac C trans ‐alleles show that pac C preferential alkaline‐expression results from derepression by depletion of the acid‐prevalent PacC 72 form. We additionally show that pac C repression requires PacX . pac X mutations suppress PacC processing recalcitrant mutations, in part, through derepressed PacC levels resulting in traces of PacC 27 formed by pH ‐independent proteolysis. pac X was cloned by impala transposon mutagenesis. PacX , with homologues within the L eotiomyceta, 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 F ot1 ‐like transposon.