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 0.5 mM, 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 sismal transduction pathway. (C) 2003 Elsevier Science (USA). All rights reserved.