Yao M C, Gall J G
Cell. 1977 Sep;12(1):121-32. doi: 10.1016/0092-8674(77)90190-8.
The macronucleus of the protozoan, Tetrahymena, is known to contain multiple rRNA genes which are not linked to the chromosomes. Here we present evidence that the germinal micronucleus of this organism contains a single gene for rRNA integrated into the chromosomal DNA. Unlike the extrachromosomal copies of the macronucleus, which are composed of a pair of reversely repeated sequences (a palindrome), the integrated copy of rDNA is nonrepetitive or half the size of the extrachromosomal rDNA. Furthermore, we have failed to detect such an integrated copy of rDNA in the macronucleus. The implications of these observations for the amplification and evolution of rDNA are discussed.
已知原生动物四膜虫的大核含有多个不与染色体相连的rRNA基因。在此，我们提供证据表明，这种生物体的生殖小核含有一个整合到染色体DNA中的rRNA单基因。与由一对反向重复序列（回文序列）组成的大核染色体外拷贝不同，rDNA的整合拷贝是非重复的，或者只有染色体外rDNA大小的一半。此外，我们未能在大核中检测到rDNA的这种整合拷贝。讨论了这些观察结果对rDNA扩增和进化的意义。
