GLOBAL REGULATORS OF RIBOSOME BIOSYNTHESIS IN YEAST

Three abundant ubiquitous DNA-binding protein factors appear to play a major role in the control of ribosome biosynthesis in yeast. Two of t hese factors mediate the regulation of transcription of ribosomal prot ein genes (rp-genes) in yeasts. Most yeast rp-genes are under transcri ptional control of Rap1p (repressor-activator protein), while a small subset of rp-genes is activated through Abf1p (ARS binding factor). Th e third protein, designated Reb1p (rRNA enhancer binding protein), whi ch binds strongly to two sites located upstream of the enhancer and th e promoter of the rRNA operon, respectively, appears to play a crucial role in the efficient transcription of the chromosomal rDNA. All thre e proteins, however, have many target sites on the yeast genome, in pa rticular, in the upstream regions of several Pol II transcribed genes, suggesting that they play a much more general role than solely in the regulation of ribosome biosynthesis. Furthermore, some evidence has b een obtained suggesting that these factors influence the chromatin str ucture and create a nucleosome-free region surrounding their binding s ites. Recent studies indicate that the proteins can functionally repla ce each other in various cases and that they act synergistically with adjacent additional DNA sequences. These data suggest that Abf1p, Rap1 p, and Reb1p are primary DNA-binding proteins that serve to render adj acent cis-acting elements accessible to specific trans-acting factors.