Modulation of calcineurin activity in Aspergillus nidulans: the roles of high magnesium concentrations and of transcriptional factor CrzA.

A proper response to elevated extracellular calcium levels helps to most organisms to keep this secondary messenger under strict control, thereby preventing inadequate activation or inhibition of many regulatory activities into cells. In fungi, the calcineurin responsive zinc-finger Crz1/CrzA transcription factor transduces calcium signaling to gene expression. In Aspergillus nidulans, absence of CrzA activity leads to alkaline pH sensitivity and loss of tolerance to high levels of extracellular calcium. Disruption of calcium uptake mechanisms or the presence of high levels of Mg2+ partially suppresses this calcium-sensitive phenotype of null crzA strain. The effects of Mg2+ on CrzA phosphorylation and perturbations that reduce calcineurin phosphatase activity on CrzA demonstrate that the calcium sensitive phenotype of null crzA strain is a consequence of up-regulated calcineurin activity under calcium-induced conditions.