Curran J F
Department of Biology, Wake Forest University, Winston-Salem, NC 27109.
Nucleic Acids Res. 1995 Feb 25;23(4):683-8. doi: 10.1093/nar/23.4.683.
tRNAs with inosine (I) in the first position read three codons ending in U, C and A. However, A-ending codons read with I are rarely used. In Escherichia coli, CGA/U/C are all read solely by tRNAICGArg. CGU and CGC are very common codons, but CGA is very rare. Three independent in vivo assays show that translation of CGA is relatively inefficient. In the first, nine tandem CGA cause a strong rho-mediated polar effect on expression of a lacZ reporter gene. The inhibition is made more extreme by a mutation in ribosomal protein S12 (rpsL), which indicates that ribosomal binding by tRNAICGArg is slow and/or unstable in the CGA cluster. The second assay, in which codons are substituted for the regulatory UGA of the RF2 frameshift, confirms that aa-tRNA selection is slow and/or unstable at CGA. In the third assay, CGA is found to be a poor 5' context for amber suppression, which suggests that an A:I base pair in the P site can interfere with translation of a codon in the A site. Two possible errors, frameshifting and premature termination by RF2, are not significant causes for inefficiency at CGA. It is concluded that the A:I pair destabilizes codon:anticodon complexes during two successive ribosomal cycles, and it is suggested that these properties contribute to the rare usage of codons read with the A:I base pair.
第一位带有次黄嘌呤（I）的tRNA能识别以U、C和A结尾的三种密码子。然而，以I识别的以A结尾的密码子很少被使用。在大肠杆菌中，CGA/U/C都仅由tRNAICGArg识别。CGU和CGC是非常常见的密码子，但CGA非常罕见。三项独立的体内试验表明，CGA的翻译效率相对较低。在第一项试验中，九个串联的CGA对lacZ报告基因的表达产生强烈的rho介导的极性效应。核糖体蛋白S12（rpsL）的突变使这种抑制作用更加极端，这表明tRNAICGArg在CGA簇中的核糖体结合缓慢和/或不稳定。第二项试验中，用密码子替代RF2移码的调控UGA，证实了在CGA处氨基酸-tRNA的选择缓慢和/或不稳定。在第三项试验中，发现CGA是琥珀抑制的不良5'上下文，这表明P位点的A:I碱基对会干扰A位点密码子的翻译。两种可能的错误，即RF2导致的移码和提前终止，不是CGA效率低下的重要原因。得出的结论是，A:I碱基对在两个连续的核糖体循环中使密码子:反密码子复合物不稳定，并且表明这些特性导致了以A:I碱基对识别的密码子的罕见使用。
