  三、 肽链合成的终止。肽链合成的终止 包括两个阶段:肽链合成的终止反应 (termination reaction)，即新合成的 肽链从最后一个tRNA的释放。终止后反应 (post-termination reaction)包括tRNA 的逐出和核糖体与mRNA的分离以及核糖 体大小亚基的分离。在已经测序的基因 中，代表C-末端氨基酸的密码子后面，总 有一个终止密码子。每一个基因中只要有   一个终止密码子就可以实现肽链合成的 终止。没有一种tRNA能识别终止密码子。 终止密码子是由蛋白质因子来识别，这与 其他密码子由tRNA来识别不同。大肠杆菌 有三种终止密码子UAG、UAA和UGA。细菌 中UAA使用频率最高，其次是UGA，UAG最 低。大肠杆菌有两种蛋白质因子催化终止 反应，称为释放因子(releasing factor， RF)。RF1识别UAA和UAG，RF2识别UGA和   UAA，RF3可刺激RF1和RF2的活性。RF作用 于核糖体的A位点，但必须在P位有肽酰 -tRNA时，才能识别A位点的终止密码子。 RF在细胞内的浓度远比起始因子和延伸 因子低，每个细胞内有600个左右，大约 相当于每10个核糖体有一个释放因子。 RF1 and RF2 recognize the termination codons and activate the ribosome to hydrolyze the peptidyl tRNA. Cleavage of polypeptide from tRNA takes place   by a reaction analogous to the usual peptidyl transfer, except that the acceptor is H2O instead of aminoacyl-tRNA.Then RF1 or RF2 is released from the ribosome by RF-3, which is a GTP-binding protein related to EF-G . RF3 resembles the GTP-binding domains of EF-Tu and EF-G, and RF1/2 resemble the C-terminal of EF-G, which mimics tRNA. This suggests that the action of RF3 on RF1/2 utilize the same site that is used by the elongation factors. The termination reaction  involves release of the completed polypeptide, but leaves a deacylated tRNA and the mRNA still associated with the ribosome. The post-termination reaction involves the dissocation of the remaining components (tRNA, mRNA, 30S and 50S subunits) requires the factor RRF, ribosome recycling factor. Like the other factors involved in release, RRF has a structure that mimics tRNA, except that it lacks an equivalent for the 3′amino acid-binding region . IF-3 is also required in this reaction. RRF acts on the 50S subunit, and IF-3 acts to remove deacaylated tRNA from the 30S subunit. Once the subunits have separated, IF-3 remains necessary, of course, to prevent their reassociation.  
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