Lowell J E, Rudner D Z, Sachs A B
Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142.
Genes Dev. 1992 Nov;6(11):2088-99. doi: 10.1101/gad.6.11.2088.
Poly(A) tail removal is the first step in the degradation pathway for some mRNAs. The purified poly(A)-binding protein (PAB)-dependent poly(A) nuclease (PAN) from yeast removes mRNA poly(A) tails in vitro by a process similar to that observed in vivo. The exonucleolytic PAN degrades poly(A) and RNA bound by PAB, and can be activated by spermidine to degrade poly(A) in the absence of PAB. The shortening of the poly(A) tail down to 10-25 nucleotides and the terminal deadenylation of this short adenine tract are kinetically distinct reactions. Poly(A) shortening rates are stimulated by the yeast a-mating factor (MFA2) RNA 3' UTR sequence, and this occurs by switching PAN from a distributive to a more processive enzyme. Terminal deadenylation rates are also stimulated to different extents by various RNAs. Inversion of the MFA2 3' UTR sequence completely inhibits the terminal deadenylation reaction owing to the presence of an inhibitory element 70 nucleotides from the poly(A) tail. Other sequence elements inserted at a similar distance from the poly(A) tail also interfere with the reaction. These data suggest that the two phases of poly(A) degradation can be regulated by mRNA sequences, and they provide a mechanistic description of how this regulation could occur in vivo.
聚腺苷酸（Poly(A)）尾的去除是某些mRNA降解途径的第一步。从酵母中纯化得到的依赖于聚腺苷酸结合蛋白（PAB）的聚腺苷酸核酸酶（PAN）在体外通过类似于体内观察到的过程去除mRNA的聚腺苷酸尾。核酸外切酶PAN降解聚腺苷酸以及与PAB结合的RNA，并且在没有PAB的情况下可被亚精胺激活以降解聚腺苷酸。聚腺苷酸尾缩短至10 - 25个核苷酸以及该短腺嘌呤序列的末端去腺苷酸化是动力学上不同的反应。酵母α-交配因子（MFA2）RNA的3'非翻译区（UTR）序列可刺激聚腺苷酸缩短速率，这是通过将PAN从一种分布性酶转变为一种更具连续性的酶来实现的。各种RNA也会不同程度地刺激末端去腺苷酸化速率。由于存在距离聚腺苷酸尾70个核苷酸的抑制元件，MFA2 3' UTR序列的倒置完全抑制了末端去腺苷酸化反应。插入到距聚腺苷酸尾相似距离处的其他序列元件也会干扰该反应。这些数据表明聚腺苷酸降解的两个阶段可由mRNA序列调节，并且它们提供了这种调节在体内如何发生的机制描述。
