TRANSCRIPTION IN THE YEAST RIBOSOMAL-RNA GENE LOCUS - DISTRIBUTION OFTHE ACTIVE GENE COPIES AND CHROMATIN STRUCTURE OF THEIR FLANKING REGULATORY, SEQUENCES

In growing yeast cells, about half of the 150 tandemly repeated rRNA g enes are transcriptionally active and devoid of nucleosomes, By using the intercalating drug psoralen as a tool to mark accessible sites alo ng chromatin DNA in vivo, we found that the active rRNA gene copies ar e rather randomly distributed along the ribosomal rRNA gene locus, Mor eover, results from the analysis of a single, tagged transcription uni t in the tandem array are not consistent with the presence of a specif ic subset of active genes that is stably maintained throughout cell di visions, In the rRNA intergenic spacers of yeast cells, an enhancer is located at the 3' end of each transcription unit, 2 kb upstream of th e next promoter, Analysis of the chromatin structure along the tandem array revealed a structural link between transcription units and adjac ent, 3' flanking enhancer sequences: each transcriptionally active gen e is flanked by a nonnucleosomal enhancer, whereas inactive, nucleosom e-packed gene copies are followed by enhancers regularly packaged in n ucleosomes. From the fact that nucleosome-free enhancers were also det ected in an RNA polymerase I mutant strain, we interpret these open ch romatin structures as being the result of specific protein-DNA interac tions that can occur before the onset of transcription, In contrast, i n this mutant strain, all of the rRNA coding sequences are packaged in nucleosomal arrays, This finding indicates that the establishment of the open chromatin conformation on the activated gene copies requires elongating RNA polymerase I molecules advancing through the template.