RIBOSOMAL-PROTEIN GENE-TRANSCRIPTION IN SACCHAROMYCES-CEREVISIAE SHOWS A BIPHASIC RESPONSE TO NUTRITIONAL CHANGES

Nutrients are major determinants of ribosomal protein (rp-) gene trans cription in Saccharomyces cerevisiae. In order to investigate the mole cular mechanisms underlying this nutritional control, yeast mutants th at display defects in the glucose upshift response of rp-gene transcri ption were isolated. Interestingly, although growth of these mutants o n glucose-containing medium was severely affected an initial increase in rp-gene transcription by nutritional upshift was still observed. Ho wever, at later time points, rp-mRNA levels decreased strongly. Variou s other types of severe growth limitation also did not prevent the ini tial upshift in transcription. The results suggest that the glucose up shift response of rp-gene transcription comprises two phases: an initi al, transient response independent of the actual growth potential, and a sustained response which is dependent on growth and requires both g lucose and adequate nitrogen sources. Previously, it was found that pr otein kinase A (Pka) mediates the initial upshift response, without th e need for regulation of Pka activity by cAMP. The present data substa ntiate that, besides the RAS/adenylate cyclase pathway, an alternative pathway through Pka regulates rp-gene transcription. In addition, evi dence is presented that the sustained response does not require Pka ac tivity. Based on these results, taken together, a model is proposed in which rp-gene transcription is dynamically regulated by multiple sign al transduction pathways.