The pH regulation of gene expression in Aspergillus nidulans is mediated by pacC, whose 678 residue‐derived protein contains three putative Cys2His2 zinc fingers. Ten pacCc mutations mimicking growth at alkaline pH remove between 100 and 214 C‐terminal residues, including a highly acidic region containing an acidic glutamine repeat. Nine pacC+/‐ mutations mimicking acidic growth conditions remove between 299 and 505 C‐terminal residues. Deletion of the entire pacC coding region mimics acidity but leads additionally to poor growth and conidiation. A PacC fusion protein binds DNA with the core consensus GCCARG. At alkaline ambient pH, PacC activates transcription of alkaline‐expressed genes (including pacC itself) and represses transcription of acid‐expressed genes. pacCc mutations obviate the need for pH signal transduction.

Many fungi grow over a wide pH range and their gene expression is tailored to the environmental pH. In Aspergillus nidulans, the transcription factor PacC, an activator of genes expressed in alkaline conditions and a repressor of those expressed in acidic conditions, undergoes two processing proteolyses, the first being pH-signal dependent and the second proteasomal. Signal transduction involves a 'go-between' connecting two complexes, one of which comprises two plasma membrane proteins and an arrestin and the other comprises PacC, a cysteine protease, a scaffold and endosomal components. The Saccharomyces cerevisiae PacC orthologue, Rim101p, differs in that it does not undergo the second round of proteolysis and it functions directly as a repressor only. PacC/Rim101-mediated pH regulation is crucial to fungal pathogenicity.

No abstract

Extremes of pH are an occupational hazard for many microorganisms. In addition to efficient pH homeostasis, survival effectively requires a regulatory system tailoring the syntheses of molecules functioning beyond the cell boundaries (permeases, secreted enzymes, and exported metabolites) to the pH of the growth environment. Our previous work established that the zinc finger PacC transcription factor mediates such pH regulation in the fungus Aspergillus nidulans in response to a signal provided by the products of the six pa/genes at alkaline ambient pH. In the presence of this signal, PacC becomes functional, activating transcription of genes expressed at alkaline pH and preventing transcription of genes expressed at acidic pH. Here we detect two forms of PacC in extracts, both forming specific retardation complexes with a PacC-binding site. Under acidic growth conditions or in acidity-mimicking pa/mutants (defective in ambient pH signal transduction), the full-length form of PacC predominates. Under alkaline growth conditions or in alkalinity-mimicking pacC c mutants (independent of the ambient pH signal), a proteolysed version containing the amino-terminal -40% of the protein predominates. This specifically cleaved shorter version is clearly functional, both as an activator for alkaline-expressed genes and as a repressor for acid-expressed genes, but the full-length form of PacC must be inactive. Thus, PacC proteolysis is an essential and pH-sensitive step in the regulation of gene expression by ambient pH. Carboxy-terminal truncations, resulting in a gain-of-function (pacC c) phenotype, bypass the requirement for the pa/signal transduction pathway for conversion of the full-length to the proteolyzed functional form.