Karrer K M, Yao M C
Department of Biology, Brandeis University, Waltham, Massachusetts 02254.
Mol Cell Biol. 1988 Apr;8(4):1664-9. doi: 10.1128/mcb.8.4.1664-1669.1988.
The extrachromosomal rRNA genes (rDNA) of Tetrahymena thermophila contain 0.4% N6-methyladenine. C3 strain rDNA was isolated, hypermethylated in vitro, and microinjected into B strain host cells. Clonal cell lines were established, and transformants were selected on the basis of resistance to paromomycin, conferred by the injected rDNA. The effects of methylation by three enzymes which methylate the sequence 5'-NAT-3', the dam, EcoRI, and ClaI methylases, were tested. Hypermethylation of the injected rDNA had no effect on transformation efficiency relative to mock-methylated controls. The injected C3 strain rDNA efficiently replaced host rDNA as the major constituent of the population of rDNA molecules. Hypermethylation of the injected DNA was not maintained through 20 to 25 cell generations.
嗜热四膜虫的染色体外核糖体RNA基因（rDNA）含有0.4%的N6-甲基腺嘌呤。分离出C3菌株的rDNA，在体外进行超甲基化处理，然后显微注射到B菌株宿主细胞中。建立了克隆细胞系，并根据注射的rDNA赋予的对巴龙霉素的抗性筛选出转化体。测试了三种对序列5'-NAT-3'进行甲基化的酶（即dam甲基化酶、EcoRI甲基化酶和ClaI甲基化酶）的甲基化作用。相对于模拟甲基化对照，注射的rDNA超甲基化对转化效率没有影响。注射的C3菌株rDNA有效地取代了宿主rDNA，成为rDNA分子群体的主要组成部分。注射DNA的超甲基化在20到25个细胞世代中未能维持。
