Visualization of the dynamic behavior of ribosomal RNA gene repeats in living yeast cells.
  Takaaki Miyazaki & Takehiko Kobayashi
Genes to Cells 16, 491-502. (2011), Article first published online: 25 APR 2011 
DOI: 10.1111/j.1365-2443.2011.01506.x

  The genes encoding ribosomal RNA, so-called rDNA, are the most abundant genes in the eukaryotic genome. They reside in tandem repetitive clusters, in some cases totaling hundreds of copies. Due to their repetitive structure, the rDNA is some of the most fragile sites in the chromosome. To investigate the dynamic nature of the repeat during the cell division cycle, we developed yeast strains in which every rDNA repeat unit has lacO arrays that associate with LacI-GFP proteins. In these strains, the entire repeat can be visualized in a living cell without the need for any special treatment. Analyses of these strains showed that the rDNA actively moved and changed shape at the boundary between the nucleolus and the nucleoplasm. At the G2/M phase, the associated sister chromatids of the rDNA formed a ring structure in the nucleolus that gave the appearance of “rotating”. We also found that the length of the rDNA repeat affected the time taken for segregation during the M phase of cell cycle.

Typical GFP-LacI signal at interphase (G1), G2/M, and M (Anaphase) in the strain with lacO-inserted rDNA.