Transcriptional regulation of phosphate-responsive genes in low-affinity phosphate-transporter-defective mutants in Saccharomyces cerevisiae.

In Saccharomyces cerevisiae, two systems have been shown to be involved in the active transport of inorganic phosphate (P(i)) across the plasma membrane, the high-affinity system and the low-affinity system. The high-affinity system consists of Pho84p and Pho89p. The low-affinity system has recently been shown to be composed of Pho87p, Pho90p, and Pho91p. In this study, we found that the Deltapho87Deltapho90Deltapho91 strain which shows repressed PHO5 expression under high-P(i) condition has, unlike the wild-type strain, increased levels of PHO5 expression at an intermediate P(i) concentration of 0.5mM, whereas it is not defective in terms of P(i) uptake under the same conditions. Moreover, we observed that the transcription levels of PHO84 and PHO89 are also increased in low-affinity P(i)-transporter-defective mutants, indicating that the inactivation of low-affinity P(i) transporters leads to the activation of the PHO pathway. In contrast to that of PHO5, PHO84, and PHO89, the transcription of PHO87, PHO90, and PHO91 genes is independent of P(i) concentration and Pho4p activity, and the increased expression level of these transporters does not occur when other transporters including PHO84 are inactivated. The fact that low-affinity P(i)-transporter-defective mutants exhibit a derepression of P(i)-responsive genes suggests that low-affinity transporters play a role not only in P(i) transport but also in the regulation of the P(i) signal transduction pathway.