Ambient pH gene regulation in fungi: making connections.

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.