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Single- and double-stranded DNA binding proteins act in concert to conserve a telomeric DNA core sequence

机译：单链和双链DNA结合蛋白共同发挥作用，以保护端粒DNA核心序列

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Background Telomeres are protective cap structures at the ends of the linear eukaryotic chromosomes, which provide stability to the genome by shielding from degradation and chromosome fusions. The cap consists of telomere-specific proteins binding to the respective single- and double-stranded parts of the telomeric sequence. In addition to the nucleation of the chromatin structure the telomere-binding proteins are involved in the regulation of the telomere length. However, the telomeric sequences are highly diverged among yeast species. During the evolution this high rate of divergency presents a challenge for the sequence recognition of the telomere-binding proteins. Results We found that the Saccharomyces castellii protein Rap1, a negative regulator of telomere length, binds a 12-mer minimal binding site (MBS) within the double-stranded telomeric DNA. The sequence specificity is dependent on the interaction with two 5 nucleotide motifs, having a 6 nucleotide centre-to-centre spacing. The isolated DNA-binding domain binds the same MBS and retains the same motif binding characteristics as the full-length Rap1 protein. However, it shows some deviations in the degree of sequence-specific dependence in some nucleotide positions. Intriguingly, the positions of most importance for the sequence-specific binding of the full-length Rap1 protein coincide with 3 of the 4 nucleotides utilized by the 3' overhang binding protein Cdc13. These nucleotides are very well conserved within the otherwise highly divergent telomeric sequences of yeasts. Conclusions Rap1 and Cdc13 are two very distinct types of DNA-binding proteins with highly separate functions. They interact with the double-stranded vs. the single-stranded telomeric DNA via significantly different types of DNA-binding domain structures. However, we show that they are dependent on coinciding nucleotide positions for their sequence-specific binding to telomeric sequences. Thus, we conclude that during the molecular evolution they act together to preserve a core sequence of the telomeric DNA.
机译：背景端粒是线性真核染色体末端的保护性帽结构，可通过屏蔽降解和染色体融合为基因组提供稳定性。该帽由端粒特异性蛋白结合端粒序列的相应单链和双链部分组成。除了染色质结构的成核外，端粒结合蛋白还参与端粒长度的调节。然而，端粒序列在酵母物种之间高度不同。在进化过程中，如此高的发散率对端粒结合蛋白的序列识别提出了挑战。结果我们发现，酿酒酵母蛋白Rap1是端粒长度的负调节剂，它与双链端粒DNA内的12-mer最小结合位点（MBS）结合。序列特异性取决于与两个5个核苷酸基序至中心的间隔的相互作用。分离的DNA结合结构域与全长Rap1蛋白结合相同的MBS，并保留相同的基序结合特征。但是，它在某些核苷酸位置显示出序列特异性依赖性程度的一些偏差。有趣的是，对于全长Rap1蛋白的序列特异性结合而言，最重要的位置与3'突出端结合蛋白Cdc13所利用的4个核苷酸中的3个一致。这些核苷酸在原本高度不同的酵母端粒序列中非常保守。结论Rap1和Cdc13是两种截然不同的DNA结合蛋白，具有高度独立的功能。它们通过明显不同类型的DNA结合结构域结构与双链与单链端粒DNA相互作用。但是，我们显示它们依赖于一致的核苷酸位置来与端粒序列进行序列特异性结合。因此，我们得出结论，在分子进化过程中，它们共同作用以保留端粒DNA的核心序列。

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