Adaptor hypothesis

Abstract: One of the steps in protein biosynthesis appears to be the attachment of each amino acid to a specific acceptor (SRNA) molecule. According to the adaptor hypothesis, each SRNA molecule would then fit to a specific complementary base sequence on a linear RNA template, specifying the sequence of amino acids in the resultant protein [1,2]. An adaptor molecule thus could have two specificities: one recognizing the correct amino acid and activating enzyme; the other, the proper position on the template. The correctness of the amino-acid sequence therefore would depend upon the precision and constancy of the adaptors. However, the structures of the enzymes and adaptors are presumably under the genetic control of the organism and might be subject to heritable modifications. It is therefore conceivable that one or both ends of an adaptor might change sufficiently to cause occasional errors and, in the long run, an alteration of the genetic code might evolve. This notion, prompted by genetic observations [3] which suggested that mutation of a bacterium might modify its translation of genetic information, lead to the present comparison of the specificities of the acceptor RNA and activating enzymes of different organisms. Several differences in specificity have been reported previously. Berg et al. [4] demonstrated that SRNA from Escherichia coli contains two distinguishable acceptors for methionine. An enzyme prepared from yeast could attach methionine to one of these, while the enzyme from E. coli could attach to both. Webster found, in pig liver, a difference between the nuclear and cytoplasmic attachment enzymes for alanine. Rendi and Ochoa [6] noted that, for leucine, the enzymes in yeast and in E. coli could attach only to their homologous SRNA. Furthermore, in the case of leucine, rat liver enzyme and SRNA were interchangeable with those from E. coli. The observations presented below show that whether the enzymes and/or acceptors from two organisms are interchangeable depends upon not only the organisms in question but also the particular amino acid

Abstract: In this, one of the most important unpublished articles in the history of science, Crick predicted the existence of adaptor molecules and enzymes for each of the twenty common amino acids. These adaptors, which were in fact found shortly thereafter and were later named transfer RNA, were essential in assembling amino acids into the polypeptide chains of which proteins consist. Moreover, Crick here introduced the concept of a "degenerate" genetic code, meaning a code in which more than one combination of DNA bases could specify a particular amino acid, as was later shown to be the case.

Abstract: The accuracy with which the genetic information contained in protein-coding genes is faithfully translated into the corresponding sequence of amino acids has long fascinated biologists. Before the mechanisms of transcription and protein synthesis had been uncovered in the exquisite molecular detail we know today, some of the inherent problems of faithful gene expression were obvious. Crick's seminal adaptor hypothesis (1) predicted the existence of many then-unknown components of translation, including the aminoacyl-tRNA synthetases. The aminoacyl-tRNA synthetases in effect define the genetic code by catalyzing a 2-step reaction that pairs amino acids with their cognate tRNAs to provide substrates for ribosomal protein synthesis. In the first step, an amino acid is condensed with ATP to form an aminoacyl-adenylate. In the second reaction, the aminoacyl group is transferred to the 3′ end of the tRNA. The aminoacyl-tRNA synthetases also provide a critical safeguard to maintain fidelity during translation of the genetic code by discriminating against and, when necessary, editing noncognate amino acids. Crick was quick to point out that specificity would be of paramount importance to the synthetases, because their function in protein synthesis would require them to precisely distinguish similar amino acids such as isoleucine and valine. Linus Pauling (2), who reasoned that small differences in binding energy between aliphatic amino acids would not provide the level of discrimination necessary for faithful protein synthesis, had also noted this particular problem in molecular recognition. This discrepancy, between the specificity achievable during recognition and the accuracy required for translation, was resolved with the discovery of editing.

Abstract: After the experimental verification of Crick's adaptor hypothesis for the role of tRNA, it became apparent that one of the most important of the protein-nucleic acid interactions occurs at the first step in protein synthesis, namely the amino acid activation reaction. It is here that a specific aminoacyl-tRNA synthetase must select, with high fidelity, a specific tRNA out of a large collection of molecules of similar size, shape, and overall composition. A mistake at this point, either by esterification of the wrong amino acid to the correct tRNA or by selection of the wrong tRNA, will inevitabley result in the insertion of an amino acid at an incorrect position in a growing polypeptide. Although there are known rules that dictate how one nucleic acid can recognize and interact with another nucleic acid, nothing is known regarding the mechanism by which a specific protein can recognize and interact with a specific nucleic acid. In order to gain some insight into the specific recognition between an aminoacyl-tRNA synthetase and its cognate tRNA, it became necessary to study the specific interaction with highly purified materials, preferably in gram quantities. An effort to do this for both the synthetases and the tRNA's was launched at the Oak Ridge National Laboratory about 6 years ago. Four high-resolution column chromatographic procedures have been developed in the ORNL Macromolecular Separations Program for the separation and production of highly purified species of tRNA's. An unexpected “spin-off” from this program is the analytical use of some of these systems to detect qualitative changes in the tRNA profile of cells as a consequence of virus infection, methionine starvation, and other metabolic alterations. Some examples of the heterologous interaction between aminoacyl-tRNA synthetases of one species with the tRNA's of another species, and some of the inherent dangers in the interpretation of such interactions, are considered. Finally, some speculations are made regarding the possible role of tRNA in regulation.