Canonical telomeres (telomerase-synthetised) are readily forming G-quadruplexes (G4) on the G-rich strand. However, there are examples of non-canonical telomeres among eukaryotes where telomeric tandem repeats are invaded by specific retrotransposons. Drosophila melanogaster represents an extreme example with telomeres composed solely by three retrotransposons—Het-A, TAHRE and TART (HTT). Even though non-canonical telomeres often show strand biased G-distribution, the evidence for the G4-forming potential is limited. Using circular dichroism spectroscopy and UV absorption melting assay we have verified in vitro G4-formation in the HTT elements of D. melanogaster. Namely 3 in Het-A, 8 in TART and 2 in TAHRE. All the G4s are asymmetrically distributed as in canonical telomeres. Bioinformatic analysis showed that asymmetric distribution of potential quadruplex sequences (PQS) is common in telomeric retrotransposons in other Drosophila species. Most of the PQS are located in the gag gene where PQS density correlates with higher DNA sequence conservation and codon selection favoring G4-forming potential. The importance of G4s in non-canonical telomeres is further supported by analysis of telomere-associated retrotransposons from various eukaryotic species including green algae, Diplomonadida, fungi, insects and vertebrates. Virtually all analyzed telomere-associated retrotransposons contained PQS, frequently with asymmetric strand distribution. Comparison with non-telomeric elements showed independent selection of PQS-rich elements from four distinct LINE clades. Our findings of strand-biased G4-forming motifs in telomere-associated retrotransposons from various eukaryotic species support the G4-formation as one of the prerequisites for the recruitment of specific retrotransposons to chromosome ends and call for further experimental studies.

中文翻译：

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端粒反转录转座子显示出在各种真核物种中形成 G-四链体的倾向

规范的端粒（端粒酶合成的）很容易在富含 G 的链上形成 G-四链体 (G4)。然而，在真核生物中存在非规范端粒的例子，其中端粒串联重复序列被特定的反转录转座子侵入。黑腹果蝇代表了一个极端的例子，其端粒仅由三个反转录转座子——Het-A、TAHRE 和 TART (HTT) 组成。尽管非规范端粒通常显示链偏向 G 分布，但 G4 形成潜力的证据是有限的。使用圆二色光谱和紫外吸收熔化测定，我们已经验证了 D. melanogaster 的 HTT 元素中的体外 G4 形成。即 Het-A 中有 3 个，TART 中有 8 个，TAHRE 中有 2 个。所有 G4 都是不对称分布的，就像典型的端粒一样。生物信息学分析表明，潜在四联体序列 (PQS) 的不对称分布在其他果蝇物种的端粒反转录转座子中很常见。大多数 PQS 位于 gag 基因中，其中 PQS 密度与更高的 DNA 序列保守性和密码子选择相关，有利于 G4 形成潜力。对来自各种真核物种（包括绿藻、双单胞菌、真菌、昆虫和脊椎动物）的端粒相关逆转录转座子的分析进一步支持了 G4 在非规范端粒中的重要性。几乎所有分析的端粒相关反转录转座子都包含 PQS，通常具有不对称链分布。与非端粒元素的比较显示了从四个不同的 LINE 进化枝中独立选择富含 PQS 的元素。