Department of Biochemistry and Molecular Biology, Thomas Jefferson University, BLSB 220, Philadelphia 19107, PA, USA.
IUBMB Life. 2010 Apr;62(4):251-60. doi: 10.1002/iub.301.
The CCA sequence is conserved at the 3' end of all mature tRNA molecules to function as the site of amino acid attachment. This sequence is acquired and maintained by stepwise nucleotide addition by the ubiquitous CCA enzyme, which is an unusual RNA polymerase that does not use a nucleic acid template for nucleotide addition. Crystal structural work has divided CCA enzymes into two structurally distinct classes, which differ in the mechanism of template-independent nucleotide selection. Recent kinetic work of the class II E. coli CCA enzyme has demonstrated a rapid and uniform rate constant for the chemistry of nucleotide addition at each step of CCA synthesis, although the enzyme uses different determinants to control the rate of each step. Importantly, the kinetic work reveals that, at each step of CCA synthesis, E. coli CCA enzyme has an innate ability to discriminate against tRNA backbone damage. This discrimination suggests the possibility of a previously unrecognized quality control mechanism that would prevent damaged tRNA from CCA maturation and from entering the ribosome machinery of protein synthesis. This quality control is relevant to cellular stress conditions that damage tRNA backbone and predicts a role of CCA addition in stress response.
CCA 序列在所有成熟 tRNA 分子的 3'端保守，作为氨基酸连接的位点。该序列通过普遍存在的 CCA 酶逐步核苷酸添加获得和维持，CCA 酶是一种不使用核酸模板进行核苷酸添加的非典型 RNA 聚合酶。晶体结构研究将 CCA 酶分为两种结构上明显不同的类别，它们在模板独立核苷酸选择的机制上有所不同。最近对 II 类大肠杆菌 CCA 酶的动力学研究表明，在 CCA 合成的每一步中，核苷酸添加的化学性质具有快速且均匀的速率常数，尽管该酶使用不同的决定因素来控制每一步的速率。重要的是，动力学研究表明，在 CCA 合成的每一步，大肠杆菌 CCA 酶都具有内在的能力来区分 tRNA 骨架损伤。这种区分表明存在一种以前未被认识的质量控制机制的可能性，该机制可以防止受损的 tRNA 进行 CCA 成熟并进入核糖体蛋白质合成机制。这种质量控制与破坏 tRNA 骨架的细胞应激条件有关，并预测了 CCA 添加在应激反应中的作用。
