Kobayashi T, Heck D J, Nomura M, Horiuchi T
National Institute for Basic Biology, Myodaijicho, Okazaki, 444-8585, Japan.
Genes Dev. 1998 Dec 15;12(24):3821-30. doi: 10.1101/gad.12.24.3821.
Saccharomyces cerevisiae carries approximately 150 copies of rDNA in tandem repeats. It was found that the absence of an essential subunit of RNA polymerase I (Pol I) in rpa135 deletion mutants triggers a gradual decrease in rDNA repeat number to about one-half the normal level. Reintroduction of the missing RPA135 gene induced a gradual increase in repeat number back to the normal level. Gene FOB1 was shown to be essential for both the decrease and increase of rDNA repeats. FOB1 was shown previously to be required for replication fork blocking (RFB) activity at RFB site in rDNA and for recombination hot-spot (HOT1) activity. Thus, DNA replication fork blockage appears to stimulate recombination and play an essential role in rDNA expansion/contraction and sequence homogenization, and possibly, in the instability of repeated sequences in general. RNA Pol I, on the other hand, appears to control repeat numbers, perhaps by stabilizing rDNA with the normal repeat numbers as a stable nucleolar structure.
酿酒酵母以串联重复的形式携带约150个rDNA拷贝。研究发现，rpa135缺失突变体中RNA聚合酶I（Pol I）的一个必需亚基的缺失会导致rDNA重复数逐渐减少至正常水平的约一半。重新引入缺失的RPA135基因会导致重复数逐渐增加至正常水平。基因FOB1被证明对rDNA重复数的减少和增加都至关重要。先前已表明FOB1是rDNA中复制叉阻断（RFB）位点的复制叉阻断（RFB）活性和重组热点（HOT1）活性所必需的。因此，DNA复制叉阻断似乎会刺激重组，并在rDNA的扩增/收缩和序列同质化中起重要作用，并且可能在一般重复序列的不稳定性中也起作用。另一方面，RNA Pol I似乎通过将具有正常重复数的rDNA稳定为稳定的核仁结构来控制重复数。
