The short-time transcriptional response of yeast cells to a mild increase in external pH (7.6) has been investigated using
DNA microarrays. A total of 150 genes increased their
mRNA level at least twofold within 45 min. Alkalinization resulted in the repression of 232 genes. The response of four upregulated genes, ENA1 (encoding a
Na+-ATPase also induced by saline stress) and PHO84, PHO89 and PHO12 (encoding genes upregulated by
phosphate starvation), was characterized further. The alkaline response of ENA1 was not affected by mutation of relevant genes involved in osmotic or oxidative signalling, but was decreased in
calcineurin and rim101 mutants. Mapping of the ENA1 promoter revealed two pH-responsive regions. The response of the upstream region was fully abolished by the
drug FK506 or mutation of CRZ1 (a
transcription factor activated by
calcium/
calcineurin), whereas the response of the downstream region was essentially
calcium independent. PHO84 and PHO12 responses were unaffected in crz1 cells, but required the presence of Pho2 and Pho4. In contrast, part of the
alkali-induced expression of PHO89 was maintained in pho4 or pho2 cells, but was fully abolished in a crz1 strain or in the presence of
FK506. Heterologous promoters carrying the minimal
calcineurin-dependent response elements found in ENA1 or FKS2 were able to drive alkaline pH-induced expression. These results demonstrate that the transcriptional response to alkaline pH involves different signalling mechanisms, and that
calcium signalling is a relevant component of this response.