Ding Wenyong, Lin Lin, Chen Bing, Dai Jianwu
Department of Biochemistry and Molecular Biology, Dalian Medical University, Dalian, China.
IUBMB Life. 2006 Dec;58(12):677-85. doi: 10.1080/15216540601034856.
Long interspersed nuclear elements 1 (L1 elements or LINE1) are the most active autonomous retrotransposons in mammalian genomes. In addition to L1 elements themselves, other protein-coding mRNAs can also be reverse transcribed and integrated into the genome through the L1-mediated retrotransposition, leading to the formation of processed pseudogenes (PPs) and retrogenes, both of which are characterized by the lack of introns and the presence of a 3' polyA tract and flanking direct repeats. PPs are unable to encode a functional protein and have accumulated frameshift mutations and premature stop codons during evolution. A few of PPs are transcriptionally active. Retrogenes preserve undisrupted coding frames and are capable of encoding a functional protein that is identical or nearly identical to that of the progenitor gene. There is a significant excess of retrogenes that originate from the X chromosome and are retrotransposed into autosomes, and most of these retrogenes are specially expressed in male germ cells, suggesting the inactivation of X-linked genes during male meiosis provides a strong selection pressure on retrogenes originating from the X chromosome.
长散在核元件1（L1元件或LINE1）是哺乳动物基因组中最活跃的自主逆转录转座子。除了L1元件本身，其他蛋白质编码mRNA也可以通过L1介导的逆转座作用进行逆转录并整合到基因组中，导致加工假基因（PPs）和反转录基因的形成，这两者的特征都是缺乏内含子、存在3'多聚腺苷酸尾以及侧翼直接重复序列。PPs无法编码功能性蛋白质，并且在进化过程中积累了移码突变和过早的终止密码子。少数PPs具有转录活性。反转录基因保留完整的编码框架，能够编码与祖先基因相同或几乎相同的功能性蛋白质。源自X染色体并逆转座到常染色体的反转录基因明显过量，并且这些反转录基因中的大多数在雄性生殖细胞中特异性表达，这表明雄性减数分裂过程中X连锁基因的失活对源自X染色体的反转录基因施加了强大的选择压力。
