Rae P M, Spear B B
Proc Natl Acad Sci U S A. 1978 Oct;75(10):4992-6. doi: 10.1073/pnas.75.10.4992.
DNA in the macronuclei of Oxytricha fallax, as in other hypotrichous ciliate protozoa, exists as small, achromosomal molecules rather than in chromosomes. We report studies on O. fallax DNA using physicochemical procedures and nucleic acid hybridization. Macronuclear DNA molecules range in size from 22 kilobase pairs (kb) to about 0.5 kb. The DNA has a buoyant density in CsCl of 1.694 g.cm(-3) and a melting temperature in 15 mM NaCl/1.5 mM sodium citrate, pH 7, at 65.4 degrees . These values correspond to 34.7% Gua + Cyt and 28.1% Gua + Cyt, respectively, and base composition determined by thin-layer chromatography of nucleotides is 32.4% Gua + Cyt. The only modified nucleotide that is detectable is N(6)-methyldeoxyadenylate (0.2%), and the amount and kind of modification cannot account for the discrepancies in nucleotide composition determination by the three methods. The genes for 25S and 19S rRNA are contained in DNA molecules 6.67 kb in length, of which at least 6.15 kb is transcribed. These rDNA molecules show no intrastrand complementarity as does rDNA in some other lower eukaryotes, and they have two asymmetric sites recognized by endonuclease EcoRI. The genes for 5S RNA are in DNA molecules 0.69 kb in length. Digestion of this DNA with restriction enzymes BamHI, BsuI, HhaI, and TaqI gives no evidence for a tandemly repeated sequence. It is likely that both 19S + 25S rRNA genes and 5S RNA genes in the Oxytricha macronucleus exist as single transcription units, and both may have "spacer" regions approximately 0.5 kb long.
与其他下毛目纤毛虫原生动物一样，扇形游仆虫大核中的DNA以小的、无染色体的分子形式存在，而非存在于染色体中。我们报告了利用物理化学方法和核酸杂交对扇形游仆虫DNA进行的研究。大核DNA分子大小范围从22千碱基对（kb）到约0.5 kb。该DNA在CsCl中的浮力密度为1.694 g.cm(-3)，在15 mM NaCl/1.5 mM柠檬酸钠、pH 7条件下的解链温度为65.4℃。这些值分别对应34.7%的鸟嘌呤+胞嘧啶和28.1%的鸟嘌呤+胞嘧啶，通过核苷酸薄层色谱法测定的碱基组成为32.4%的鸟嘌呤+胞嘧啶。唯一可检测到的修饰核苷酸是N(6)-甲基脱氧腺苷酸（0.2%），修饰的量和种类无法解释三种方法在核苷酸组成测定上的差异。25S和19S rRNA的基因包含在长度为6.67 kb的DNA分子中，其中至少6.15 kb被转录。这些rDNA分子不像其他一些低等真核生物的rDNA那样具有链内互补性，并且它们有两个被内切酶EcoRI识别的不对称位点。5S RNA的基因存在于长度为0.69 kb的DNA分子中。用限制性酶BamHI、BsuI、HhaI和TaqI消化该DNA未发现串联重复序列的证据。扇形游仆虫大核中的19S + 25S rRNA基因和5S RNA基因可能都作为单个转录单位存在，并且两者可能都有大约0.5 kb长的“间隔”区域。
