一种与拓扑异构酶I类似的位点和链特异性核酸酶活性构成了四膜虫的rRNA基因。

Gocke E, Bonven B J, Westergaard O

Nucleic Acids Res. 1983 Nov 25;11(22):7661-78. doi: 10.1093/nar/11.22.7661.

Exposure of macronuclear chromatin from Tetrahymena thermophila to sodium dodecyl sulfate causes an endogenous nuclease to cleave the extra-chromosomal rDNA at specific sites. All cuts are single-strand cleavages specific to the non-coding strand. Three cleavages map in the central non-transcribed spacer of the palindromic molecule at positions -1000, -600 and -150 bp with respect to the transcription initiation point. A fourth site is located close to the transcription termination point, while no cleavage is observed in the coding region. The position of each cleavage is in the immediate neighbourhood of DNAse I hypersensitive sites. Additionally, certain DNA sequence motifs are repeated in the region around the cleavages. Upon cleavage induction a protein becomes attached to the rDNA. Our results indicate covalent binding to the generated 3' end, in analogy to the aborted reaction of topoisomerase I.

嗜热四膜虫的大核染色质暴露于十二烷基硫酸钠会使一种内源性核酸酶在特定位点切割染色体外的rDNA。所有切割都是针对非编码链的单链切割。三个切割位点位于回文分子中央非转录间隔区，相对于转录起始点分别位于-1000、-600和-150 bp处。第四个位点靠近转录终止点，而在编码区未观察到切割。每个切割位点都紧邻DNase I超敏位点。此外，切割位点周围区域存在某些重复的DNA序列基序。切割诱导后，一种蛋白质会附着在rDNA上。我们的结果表明该蛋白质与产生的3'末端共价结合，类似于拓扑异构酶I的流产反应。