Abstract: Summary The expression of genes of Saccharomyces cerevisiae encoding the enzymes involved in the metabolism of thiamin (THI genes) is co-ordinately repressed by exogenous thiamin and induced in the absence of thiamin. In this yeast THI regulatory system acts mainly at the transcriptional level, thiamin pyrophosphate (TDP) seems to serve as a corepressor, and genetic studies have identified three positive regulatory factors (Thi2p, Thi3p and Pdc2p). We found in a DNA microarray analysis that the expression of THI genes increased 10- to 90-fold in response to thiamin deprivation, and likewise, the expression of THI2 and THI3 increased 17-fold and threefold, respectively. After transfer from repressing to inducing medium, the promoter activity of both THI2 and THI3 increased in parallel with that of PHO3, one of THI genes. The stimulation of THI3 promoter activity was diminished by deletion of THI3, indicative of the autoregulation of THI3. The THI genes were not induced when THI2 was expressed from the yeast GAL1 promoter in a thi3Delta strain or when THI3 was expressed in a thi2Delta strain, suggesting that Thi2p and Thi3p participate simultaneously in the induction. When mutant Thi3p proteins lacking TDP-binding activity were produced in the thi3Delta strain, THI genes were expressed even under thiamin-replete conditions. This result supports the hypothesis that Thi3p senses the intracellular signal of the THI regulatory system to exert transcriptional control. Furthermore, Thi2p and Thi3p were demonstrated to bind each other and this interaction was partially diminished by exogenous thiamin, suggesting that Thi2p and Thi3p stimulate the expression as a complex whose function is disturbed by TDP bound to Thi3p. We discuss the possibility that the induction of THI genes is triggered by the activation of the complex attributed to decrease in intracellular TDP and the elevated complex in the autoregulatory fashion further upregulates THI genes. This is the first report of the involvement of the TDP-binding motif in genetic regulation.