Two zinc finger transcription factors, CrzA and SltA, are involved in cation homoeostasis and detoxification in Aspergillus nidulans

To investigate cation adaptation and homoeostasis in Aspergillus nidulans, two transcription-factor-encoding genes have been characterized. The A. nidulans orthologue crzA of the Saccharomyces cerevisiae CRZ1 gene, encoding a transcription factor mediating gene regulation by Ca²⁺, has been identified and deleted. The crzA deletion phenotype includes extreme sensitivity to alkaline pH, Ca²⁺ toxicity and aberrant morphology connected with alterations of cell-wall-related phenotypes such as reduced expression of a chitin synthase gene, chsB. A fully functional C-terminally GFP (green fluorescent protein)-tagged form of the CrzA protein is apparently excluded from nuclei in the absence of added Ca²⁺, but rapidly accumulates in nuclei upon exposure to Ca²⁺. In addition, the previously identified sltA gene, which has no identifiable homologues in yeasts, was deleted, and the resulting phenotype includes considerably enhanced toxicity by a number of cations other than Ca²⁺ and also by alkaline pH. Reduced expression of a homologue of the S. cerevisiae P-type ATPase Na⁺ pump gene ENA1 might partly explain the cation sensitivity of sltA-null strains. Up-regulation of the homologue of the S. cerevisiae vacuolar Ca²⁺/H⁺ exchanger gene VCX1 might explain the lack of Ca²⁺ toxicity to null-sltA mutants, whereas down-regulation of this gene might be responsible for Ca²⁺ toxicity to crzA-null mutants. Both crzA and sltA encode DNA-binding proteins, and the latter exerts both positive and negative gene regulation.