Nielsen L, Edström J E
Department of Molecular Genetics, University of Lund, Sweden.
Mol Cell Biol. 1993 Mar;13(3):1583-9. doi: 10.1128/mcb.13.3.1583-1589.1993.
The dipteran Chironomus tentans has complex tandemly repeated 350-bp DNA sequences at or near the chromosome ends. As in Drosophila melanogaster, short simple repeats with cytosines and guanines in different strands have never been observed. We were therefore interested in learning whether the Chironomus repeats could have evolved from simple sequence telomeric DNA, which might suggest that they constitute a functional equivalent. We screened for repeat units with evolutionarily ancient features within the tandem arrays and recovered two clones with a less-evolved structure. Sequence analysis reveals that the present-day 350-bp unit probably evolved from a simpler 165-bp unit through the acquisition of transposed sequences. The 165-bp unit contains DNA with a highly biased distribution of cytosine and guanine between the two strands, although with the ratios inverted in two minor parts of the repeat. It is largely built up of short degenerate subrepeats for which most of the sequence can be reconstructed. The consensus for the subrepeat sequence is similar to the simple telomeric repeat sequences of several kinds of eukaryotes. We propose that the present-day unit has evolved from telomeric, simple sequence, asymmetric DNA from which it has retained some original sequence features and possibly functions.
双翅目昆虫摇蚊（Chironomus tentans）在染色体末端或其附近具有复杂的串联重复350碱基对DNA序列。与黑腹果蝇（Drosophila melanogaster）一样，从未观察到不同链上含有胞嘧啶和鸟嘌呤的短简单重复序列。因此，我们有兴趣了解摇蚊的重复序列是否可能从简单序列端粒DNA进化而来，这可能表明它们构成了功能等同物。我们在串联阵列中筛选具有进化上古老特征的重复单元，并获得了两个结构进化程度较低的克隆。序列分析表明，当今的350碱基对单元可能是通过获得转座序列从一个更简单的165碱基对单元进化而来的。165碱基对单元包含两条链之间胞嘧啶和鸟嘌呤分布高度偏向的DNA，尽管在重复序列的两个小部分中比例颠倒。它主要由短的简并亚重复序列组成，其大部分序列可以重建。亚重复序列的共有序列类似于几种真核生物的简单端粒重复序列。我们提出，当今的单元是从端粒、简单序列、不对称DNA进化而来的，它保留了一些原始序列特征和可能的功能。
