Ribosomal protein gene transcription in Saccharomyces cerevisiae shows a biphasic response to nutritional changes

Nutrients are major determinants of ribosomal protein (rp-) gene transcription in Saccharomyces cerevisiae. In order to investigate the molecular mechanisms underlying this nutritional control, yeast mutants that display defects in the glucose upshift response of rp-gene transcription were isolated. Interestingly, although growth of these mutants on glucose-containing medium was severely affected an initial increase in rp-gene transcription by nutritional upshift was still observed. However, at later time points, rp-mRNA levels decreased strongly. Various other types of severe growth limitation also did not prevent the initial upshift in transcription. The results suggest that the glucose upshift response of rp-gene transcription comprises two phases: an initial, transient response independent of the actual growth potential, and a sustained response which is dependent on growth and requires both glucose and adequate nitrogen sources. Previously, it was found that protein kinase A (Pka) mediates the initial upshift response, without the need for regulation of Pka activity by cAMP. The present data substantiate that, besides the RAS/adenylate cyclase pathway, an alternative pathway through Pka regulates rp-gene transcription. In addition, evidence is presented that the sustained response does not require Pka activity. Based on these results, taken together, a model is proposed in which rp-gene transcription is dynamically regulated by multiple signal transduction pathways.