Regulation of gene expression by ambient pH in Aspergillus: genes expressed at acid pH.

Ambient pH strongly influences the synthesis of secreted enzymes, permeases and exported metabolites by a wide range of micro-organisms. For example, numerous microbes secrete acid phosphatase in acidic environments and alkaline phosphatase in alkaline environments. Only in the ascomycete Aspergillus nidulans, however, is characterization [ 1-51 of the pH-regulatory system sufficiently advanced to have enabled construction of a comprehensive model covering both acidand alkaline-expressed structural genes [ S ] . Briefly, the DNA-binding region of the pacC-encoded transcription factor (PacC) contains three Cys2His2 zinc fingers. When converted into a functional form at alkaline ambient pH in response to the ambient pH signal transduced by the (six) pal gene product pathway, PacC activates expression of genes expressed in acidic growth conditions (such as that encoding acid phosphatase). PacC is itself an alkalineexpressed gene subject to autogenous regulation [S] . pacC' mutations, mimicking alkaline growth conditions, remove a C-terminal highly acidic region, containing an acidic glutamine repeat, responsible for negatively modulating PacC function [5,6]. These pacC' mutations obviate the need for pH signalling and therefore constitute a gain-of-function class [ 1,4-61. More severe truncation mutations, designated pacC +'-, constitute a partial loss-of-function class and mimic acid growth conditions [5,6]. A null mutant, deleted for the entire pace coding region, mimics acidic growth conditions more extremely and, in addition, leads to poor growth and conidiation [S]. Gel mobility shifts and DNase I footprinting have identified three single and one double PacCbinding sites in the promoter for the isopenicillin N synthetase (ipd) gene, enabling a consensus core-binding sequence Sr-GCCARG-3' to be identified and confirmed by cross-competition experiments involving a single base-pair change ~ 1 .