Rapid changes in transcription and chromatin structure of ribosomal genes in yeast during growth phase transitions

Transcription of ribosomal genes is coordinated with cellular growth. Changes in transcription may be influenced by an alteration in the number of active ribosomal genes and/or a change in the transcription rate of active genes. We measured changes in rDNA transcription during growth phase transitions in the yeast Saccharomyces cerevisiae and the concomitant changes in chromatin structure of the ribosomal genes. A quantitative transcription run-on (TRO) assay was developed to monitor transcription of ribosomal genes, and rDNA chromatin was separated into active (non-nucleosomal) and inactive (nucleosomal) genes using psoralen photo-crosslinking. TRO indicates that transcription levels of ribosomal genes drop dramatically as cells enter stationary phase, but are rapidly restored when cells are diluted into fresh medium. However, changes in the proportion of active genes during these transitions, although equally rapid, represented only a small fraction of the total rDNA. We conclude that changes in rDNA chromatin structure are temporally coordinated with growth rate, but quantitatively insufficient to account for changes in transcription. These results support the model that regulation of rRNA synthesis occurs mainly by altering the transcription rate of active ribosomal genes, and changes in the number of active rDNA gene copies contribute much less to this regulation.