Borusenius S N, Belozerskaia N A, Merkulova N A, Vorob'ev V I
Mol Biol (Mosk). 1978 May-Jun;12(3):676-88.
Reassociation kinetics of macronuclear (Ma) DNA of the ciliate Tetrahymena pyriformis GL has been studied. The genome size calculated by the kinetic complexity of DNA constitutes of 2.0.10(8) base pairs, which corresponds to the molecular weight of 1.2.10(11) dalton. About 90% of the Ma DNA fragments of 200-300 base pairs in length reassociate at a rate corresponding to the unique sequences and 3--5% of the nucleotide sequences at a ten times exceeding rate. About 3--5% of the genome consists of sequences reassociating at Cot practically equal to zero. Most of the zero time reassociating DNA seems to be represented by inverted nucleotide sequences. This conclusion was drawn from the study of the at zero time reassociating DNA isolated preparatively by hydroxyapatite chromatography. This fraction shows an approximately 75% resistance to S1-nuclease treatment which is independent from the original DNA concentration. Heat denaturation and reassociation are mutually reversible yielding hyper- and hypochromic effects. The majority of the inverted sequences are likely to be unique, about 20% is repeated scores of time in the Tetrahymena genome. This assumption has been made on the strength of reassociation of the zero time [125I]DNA fraction (after breaks in single-strand hair pin loop with the aid of S1-nuclease) with non-fractionated Ma DNA. According to the equilibrium distribution in a CsCl density gradient the mean nucleotide content of inverted [14C]DNA sequences does not differ from that of non-fractionated DNA. Consequently the largest part of the isolated fraction of inverted nucleotide sequences of the Tetrahymena genome are not fragments of ribosomal genes. The evidence of reassociation kinetics of Ma DNA fragments of various lengths allows to suggest that inverted sequences from large blocks (no less than 6000 base pairs) and scores of times repeated sequences may alternate with the regions of unique sequences.
对梨形四膜虫GL大核（Ma）DNA的重缔合动力学进行了研究。通过DNA动力学复杂度计算得出的基因组大小为2.0×10⁸个碱基对，对应分子量为1.2×10¹¹道尔顿。约90%长度为200 - 300个碱基对的Ma DNA片段以对应单一序列的速率重缔合，3 - 5%的核苷酸序列以快十倍的速率重缔合。约3 - 5%的基因组由在Cot值几乎为零的情况下重缔合的序列组成。大部分零时间重缔合DNA似乎由反向核苷酸序列代表。这一结论是通过对用羟基磷灰石色谱法制备分离的零时间重缔合DNA的研究得出的。该部分对S1核酸酶处理表现出约75%的抗性，且与原始DNA浓度无关。热变性和重缔合是相互可逆的，产生增色和减色效应。大多数反向序列可能是单一的，约20%在四膜虫基因组中重复数十次。这一假设是基于零时间[¹²⁵I]DNA部分（在借助S1核酸酶将单链发夹环打断后）与未分级的Ma DNA的重缔合得出的。根据在CsCl密度梯度中的平衡分布，反向[¹⁴C]DNA序列的平均核苷酸含量与未分级DNA的平均核苷酸含量没有差异。因此，四膜虫基因组中分离出的反向核苷酸序列部分的最大部分不是核糖体基因的片段。不同长度的Ma DNA片段重缔合动力学的证据表明，来自大的片段（不少于6000个碱基对）的反向序列和重复数十次的序列可能与单一序列区域交替出现。
