Fahy Deirdre, Conconi Antonio, Smerdon Michael J
Biochemistry and Biophysics, School of Molecular Biosciences, Washington State University, Pullman, WA 99164-4660, USA.
Exp Cell Res. 2005 May 1;305(2):365-73. doi: 10.1016/j.yexcr.2005.01.016.
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.
核糖体基因的转录与细胞生长相协调。转录的变化可能受到活性核糖体基因数量的改变和/或活性基因转录速率变化的影响。我们测量了酿酒酵母生长阶段转变过程中rDNA转录的变化以及核糖体基因染色质结构的相应变化。开发了一种定量转录延伸（TRO）测定法来监测核糖体基因的转录，并使用补骨脂素光交联将rDNA染色质分离为活性（非核小体）和非活性（核小体）基因。TRO表明，随着细胞进入稳定期，核糖体基因的转录水平急剧下降，但当细胞稀释到新鲜培养基中时，转录水平会迅速恢复。然而，在这些转变过程中活性基因比例的变化虽然同样迅速，但仅占总rDNA的一小部分。我们得出结论，rDNA染色质结构的变化在时间上与生长速率相协调，但在数量上不足以解释转录的变化。这些结果支持了这样一种模型，即rRNA合成的调节主要通过改变活性核糖体基因的转录速率来实现，而活性rDNA基因拷贝数的变化对这种调节的贡献要小得多。
