The yeast QDR3 gene encodes a plasma membrane drug : H + antiporter of the DHA1 family that was described as conferring resistance against the drugs quinidine, cisplatin and bleomycin and the herbicide barban, similar to its close homologue QDR2. In this work, a new physiological role for Qdr3 in polyamine homeostasis is proposed. QDR3 is shown to confer resistance to the polyamines spermine and spermidine, but, unlike Qdr2, also a determinant of resistance to polyamines, Qdr3 has no apparent role in K + homeostasis. QDR3 transcription is upregulated in yeast cells exposed to spermine or spermidine dependent on the transcription factors Gcn4, which controls amino acid homeostasis, and Yap1, the main regulator of oxidative stress response. Yap1 was found to be a major determinant of polyamine stress resistance in yeast and is accumulated in the nucleus of yeast cells exposed to spermidine-induced stress. QDR3 transcript levels were also found to increase under nitrogen or amino acid limitation; this regulation is also dependent on Gcn4. Consistent with the concept that Qdr3 plays a role in polyamine homeostasis, QDR3 expression was found to decrease the intracellular accumulation of [ 3 H]spermidine, playing a role in the maintenance of the plasma membrane potential in spermidine-stressed cells. INTRODUCTIONMultidrug efflux transporters are found in all living cells and are thought to recognize a wide variety of structurally and pharmacologically unrelated drugs. They are proposed to actively extrude or compartmentalize drugs and other xenobiotics, thus providing protection from these compounds (Jungwirth & Kuchler, 2006;Paulsen, 2003;Prasad et al., 2002;Roepe et al., 1996;. The activity of these proteins underlies the manifestation of cellular drug resistance, seriously limiting the therapeutic potential of drugs (Hayes & Wolf, 1997). The in silico analysis of the yeast genome revealed the existence of 23 putative drug : H + antiporters of the multiple drug resistance (MDR) family of the major facilitator superfamily (MFS). Although many of these proteins have been shown to confer resistance to a wide variety of drugs and chemicals (Paulsen et al., 1998;, the molecular mechanisms behind their apparent promiscuity remain elusive and a topic of debate (Jungwirth & Kuchler, 2006;Paulsen, 2003;Prasad et al., 2002;Roepe et al., 1996;.In the present work, we have further examined the biological function of the MFS-MDR transporter encoded by the QDR3 gene, which is present in the plasma membrane of Saccharomyces cerevisiae (Huh et al., 2003;Tenreiro et al., 2005). Qdr3 belongs to cluster I of the DHA1 drug efflux family, including putative drug : H + antiporters with 12 predicted membrane-spanning segments (Paulsen et al., 1998), and confers yeast resistance against the antiarrhythmic and antimalarial drug quinidine, the herbicide barban and the antitumour agents bleomycin and cisplatin (Tenreiro et al., 2005). Through genome-wide screenings, QDR3 gene deletion was also found to increase yeast susceptibility towards ma...