Short interspersed elements (SINEs) are highly abundant components of mammalian genomes that are propagated by retrotransposition. SINEs are recognized as a causal agent of human disease and must also have had a profound influence in shaping eukaryotic genomes. The B2 SINE family constitutes approximately 0.7% of total mouse genomic DNA (ref. 2) and is also found at low abundance in humans. It resembles the Alu family in several respects, such as its mechanism of propagation. B2 SINEs are derived from tRNA and are transcribed by RNA polymerase (pol) III to generate short transcripts that are not translated. We find here, however, that one B2 SINE also carries an active pol II promoter located outside the tRNA region. Indeed, a B2 element is responsible for the production of a mouse Lama3 transcript. The B2 pol II promoters can be bound and stimulated by the transcription factor USF (for upstream stimulatory factor), as shown by transient transfection experiments. Moreover, this pol II activity does not preclude the pol III transcription necessary for retrotransposition. Dispersal of B2 SINEs by retrotransposition may therefore have provided numerous opportunities for creating regulated pol II transcription at novel genomic sites. This mechanism may have allowed the evolution of new transcription units and new genes.

中文翻译：

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转座的B2 SINE元件可以提供可移动的RNA聚合酶II启动子。

短穿插元件（SINE）是哺乳动物基因组中高度丰富的组成部分，通过逆转座子繁殖。SINE被认为是人类疾病的病因，并且在塑造真核基因组方面也必须具有深远的影响。B2 SINE家族约占小鼠基因组DNA总量的0.7％（参考文献2），并且在人类中的含量也很低。它在某些方面类似于Alu家族，例如其传播机制。B2 SINEs源自tRNA，并通过RNA聚合酶（pol）III转录，生成不被翻译的短转录本。但是，我们在这里发现一个B2 SINE也带有位于tRNA区域之外的活性pol II启动子。实际上，B2元素负责产生小鼠Lama3转录本。如瞬时转染实验所示，B2 pol II启动子可以被转录因子USF（用于上游刺激因子）结合和刺激。此外，这种pol II活性并不排除逆转录所需的pol III转录。因此，通过逆转座来分散B2 SINE可能为在新的基因组位点产生调控的pol II转录提供了许多机会。这种机制可能已经允许新的转录单位和新基因的进化。因此，通过逆转座来分散B2 SINE可能为在新的基因组位点产生调控的pol II转录提供了许多机会。这种机制可能允许新的转录单位和新基因的进化。因此，通过逆转座来分散B2 SINE可能为在新的基因组位点产生调控的pol II转录提供了许多机会。这种机制可能已经允许新的转录单位和新基因的进化。