短散在核元件
短散在核元件（Short interspersed nuclear elements，简称SINE）是真核生物基因组中长100至700bp的转位子序列[1]，属于反转录转座子，哺乳类基因组约有13%为SINE[2]。SINE与LINE等转位子皆是“寄生”于生物基因组中的DNA元件，某些情况下可能对生物体有害，但有时细胞也将其用来调控自己的基因表现[3]。
SINE一般不具有长末端重复序列（LTR）[4]，其序列从5端至3端可大致分为头端、中端与尾端三部分，其中头端大多来自tRNA等由RNA聚合酶III转录的RNA[5]，例如Alu元件的5端序列即来自7SL RNA[6]；中端序列常与长散在核元件（LINE）相似，因此可利用后者编码的内切酶；尾端则是由一些重复序列组成[7]。
SINE头端的序列与tRNA等小RNA相似，可被RNA聚合酶III转录[8]，且其启动子和tRNA一样是转录序列的一部份（称为“内部启动子”），而非位于其上游[9]。SINE转录出的mRNA不编码任何蛋白质，相较之下LINE可编码具内切酶和反转录酶活性的蛋白，SINE因具有和LINE同源的序列[10]，其RNA跟LINE的RNA均可利用这些蛋白，反转录成DNA后再随机插入基因组中[4][11]。SINE因需仰赖基因组中的其他序列编码的酵素才能增殖，属于非自主的逆转录转座子（non-autonomous retrotransposon），而LINE则是自主的逆转录转座子（autonomous retrotransposon）[12]。LINE的转录在生殖系细胞和早期胚胎中较为活跃，因此SINE也大多在这些阶段转录增殖，胚胎发育后期以后，体细胞中SINE的转录一般会被抑制，不过某些状况下可能使其恢复表现[13]。
Alu元件是灵长类演化过程中相当重要的SINE，约长300bp，人类基因组中有超过百万个Alu元件，占基因组大小超过10%[14]，许多其他动物基因组中也有大量的SINE[15]。SINE插入重要的基因序列可影响基因表现而造成人类的遗传疾病[13][16] ，例如有些乳癌、大肠癌、白血病、血友病、囊肿性纤维化、神经纤维瘤病与Dent病为Alu元件插入造成[4]。
参考文献
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