Intrinsic Precision of Aminoacyl-tRNA Synthesis Enhanced through Parallel Systems of Ligands

Abstract

MOST aminoacyl-tRNAs possessed by an organism must contain amino-acids matched to their correct anticodon so that the meaning of structural genetic information will be preserved. It is only recently, however, that we have begun to understand the underlying molecular mechanisms with regard to isoleucyl-tRNA of Escherichia coli B. Isoleucyl-tRNA synthetase, which is representative of many others in size and other properties¹, is quite selective among tRNAs, in that it binds strongly only to the cognate tRNA^Ile species². However, a weaker, but still significant affinity for non-cognate tRNAs from E. coli can be detected³. In addition, non-cognates are isoleucylated (ref. 3 and unpublished work), albeit at a maximum rate considerably slower than tRNA^Ile. Two of these reactions, the binding and isoleucylation of tRNA^phe (E. coli)³ and of tRNA_f^Met (E. coli), have been studied in detail. The generality of this phenomenon could prove important. First, the tRNA concentrations in E. coli are high (they can be no less than about 0.5 × 10⁻⁵ M^4,5 for individual species) compared with those usual in vitro and thus even weak binding could be significant. Second, many incorrect interactions are possible. Assuming that there are about sixty molecular species of tRNA and twenty species of aminoacyl-tRNA synthetase, there are 1,200 possible interacting pairs, and only about sixty of these (or 5%) are cognates. Since it is presumably desirable that misacylated tRNAs be held to a very small fraction of the total, misacylation could be significant, even if it is always a slow reaction. I have, as of writing, examined five species of purified tRNA, of which the two already mentioned give misacylations which are sufficiently facile to be easily studied under usual in vitro conditions. The other three are much less easily isoleucylated, but also give indications of reaction (my unpublished data). Thus, this limited survey emphasizes that these reactions may be common, and that rejection of non-cognate tRNAs by the aminoacyl-tRNA synthetase may not be the only mechanism by which the correctness of the aminoacyl-tRNAs is assured. In fact, I have already reported^3,6 that Ile-tRNA^phe, synthesized by isoleucyl-tRNA synthetase, is rapidly destroyed by phenylal-anyl-tRNA synthetase, and have suggested⁶ that the aminoacyl-tRNA synthetases may have a function in addition to synthesis of aminoacyl-tRNAs; that of destruction of misacylated cognate tRNAs.