Activation of transcription by metabolic intermediates of the pyrimidine biosynthetic pathway

Saccharomyces cerevisiae responds to pyrimidine starvation by increasing th e expression of four URA genes, encoding the enzymes of de novo pyrimidine biosynthesis, three- to eightfold. The increase in gene expression is depen dent on a transcriptional activator protein, Ppr1p. Here, we investigate th e mechanism by which the transcriptional activity of Ppr1p responds to the level of pyrimidine biosynthetic intermediates. We find that purified Ppr1p is unable to promote activation of transcription in an in vitro system. Tr anscriptional activation by Ppr1p can be observed, however, if either dihyd roorotic acid (DHO) or erotic acid (OA) is included in the transcription re actions. The transcriptional activation function and the DHO/OA-responsive element of Ppr1p localize to the carboxyl-terminal 134 amino acids of the p rotein. Thus, Ppr1p directly senses the level of early pyrimidine biosynthe tic intermediates within the cell and activates the expression of genes enc oding proteins required later in the pathway. These results are discussed i n terms of (i) regulation of the pyrimidine biosynthetic pathway and (ii) a novel mechanism of regulating gene expression.