Pardue Mary-Lou, Debaryshe Pg
Department of Biology; Massachusetts Institute of Technology; Cambridge, MA USA.
Mob Genet Elements. 2011 Jul;1(2):128-134. doi: 10.4161/mge.1.2.16914. Epub 2011 Jul 1.
Drosophila telomeres are remarkable because they are maintained by telomere-specific retrotransposons, rather than the enzyme telomerase that maintains telomeres in almost every other eukaryotic organism. Successive transpositions of the Drosophila retrotransposons onto chromosome ends produce long head-to-tail arrays that are analogous in form and function to the long arrays of short repeats produced by telomerase in other organisms. Nevertheless, Drosophila telomere repeats are retrotransposons, complex entities three orders of magnitude longer than simple telomerase repeats. During the >40-60 My they have been coevolving with their host, these retrotransposons perforce have evolved a complex relationship with Drosophila cells to maintain populations of active elements while carrying out functions analogous to those of telomerase repeats in other organisms. Although they have assumed a vital role in maintaining the Drosophila genome, the three Drosophila telomere-specific elements are non-LTR retrotransposons, closely related to some of the best known non-telomeric elements in the Drosophila genome. Thus, these elements offer an opportunity to study ways in which retrotransposons and their host cells can coevolve cooperatively. The telomere-specific elements display several characteristics that appear important to their roles at the telomere; for example, we have recently reported that they have evolved at least two innovative mechanisms for protecting essential sequence on their 5'ends. Because every element serves as the end of the chromosome immediately after it transposes, its 5'end is subject to chromosomal erosion until it is capped by a new transposition. These two mechanisms make it possible for at least a significant fraction of elements to survive their initial time as the chromosome end without losing sequence necessary to be competent for subsequent transposition. Analysis of sequence from >90 kb of assembled telomere array shows that these mechanisms for small scale sequence protection are part of a unified set which maintains telomere length homeostasis. Here we concentrate on recently elucidated mechanisms that have evolved to provide this small scale 5' protection.
果蝇的端粒很特别，因为它们是由端粒特异性逆转座子维持的，而不是由几乎在所有其他真核生物中维持端粒的端粒酶。果蝇逆转座子连续转座到染色体末端产生了长的头对头排列，其形式和功能类似于其他生物中端粒酶产生的短重复序列的长排列。然而，果蝇端粒重复序列是逆转座子，是比简单的端粒酶重复序列长三个数量级的复杂实体。在超过4000万至6000万年的时间里，这些逆转座子与它们的宿主共同进化，它们必然与果蝇细胞进化出了复杂的关系，以维持活跃元件群体，同时执行与其他生物中端粒酶重复序列类似的功能。尽管它们在维持果蝇基因组方面发挥了至关重要的作用，但这三种果蝇端粒特异性元件是非LTR逆转座子，与果蝇基因组中一些最著名的非端粒元件密切相关。因此，这些元件提供了一个机会来研究逆转座子及其宿主细胞如何协同共同进化。端粒特异性元件表现出一些对它们在端粒中的作用似乎很重要的特征；例如，我们最近报道它们已经进化出至少两种创新机制来保护其5'端的基本序列。因为每个元件在转座后立即作为染色体末端，其5'端会受到染色体侵蚀，直到被新的转座所覆盖。这两种机制使得至少相当一部分元件能够在作为染色体末端的初始阶段存活下来，而不会丢失后续转座所需的序列。对超过90kb的组装端粒阵列序列的分析表明，这些小规模序列保护机制是维持端粒长度稳态的统一机制的一部分。在这里，我们集中讨论最近阐明的为提供这种小规模5'保护而进化的机制。
