TRANSCRIPTION ACTIVATION OF YEAST RIBOSOMAL-PROTEIN GENES REQUIRES ADDITIONAL ELEMENTS APART FROM BINDING-SITES FOR ABF1P OR RAP1P

All ribosomal protein (rp) gene promoters from Saccharomyces cerevisia e studied so far contain either (usually two) binding sites for the gl obal gene regulator Rap1p or one binding site for another global facto r, Abf1p. Previous analysis of the rpS33 and rpL45 gene promoters sugg ested that apart from the Abf1 binding site, additional cis-acting ele ments play a part in transcription activation of these genes. We desig ned a promoter reconstruction system based on the beta-glucuronidase r eporter gene to examine the role of the Abf1 binding site and other pu tative cis-acting elements in promoting transcription. An isolated Abf 1 binding site turned out to be a weak activating element. A T-rich se quence derived from the rpS33 proximal promoter was found to be strong er, but full transcription activation was only achieved by a combinati on of these elements. Both in the natural rpL45 promoter and in the re constituted promoter, a Rap1 binding site could functionally replace t he Abf1 binding site. Characteristic rp gene nutritional control of tr anscription, evoked by a carbon source upshift or by nitrogen re-feedi ng to nitrogen starved cells, could only be mediated by the combined A bf1 (or Rap1) binding site and T-rich element and not by the individua l elements. These results demonstrate that Abf1p and Rap1p do not acti vate rp genes in a prototypical fashion, but rather may serve to poten tiate transcription activation through the T-rich element.