Adaptor hypothesis

Figure 4.29 The adaptor hypothesis. (From Voet and Voet, 2004. Reproduced with permission from John Wiley Sons., Inc.)...

FIGURE 27-2 Crick s adaptor hypothesis. Today we know that the amino acid is covalently bound at the 3 end of a tRNA molecule and that a specific nucleotide triplet elsewhere in the tRNA interacts with a particular triplet codon in mRNA through hydrogen bonding of complementary bases. 

Classic experiments providing proof for Crick s adaptor hypothesis and showing that amino acids are not checked after they are linked to tRNAs. 

M. Ibba, H.D. Becker, C. Stathopoulos, D.L. Tumbula, and D. Soil. 2000. The adaptor hypothesis revisited Trends Biochem. Sci. 25 311-316. (PubMed)... 

Codon 1 Codon 2 Codon 3 FIGURE 3.33 The adaptor hypothesis. (Adapted from Voet Voet, 2004.)... 

Historical. The G.c. was cracked between 1961 and 1963, primarily by Khorana, Matthaei, Nirenberg and Ochoa, by the analysis of the translation products of synthetic copolymers of pyrimidine and purine nucleotides. Brenner and Crick showed that defined chemical changes in bacteriophage DNA produced certain types of mutations, which were in accordance with the proposed code. Comparison of the primary sequences of viral coat proteins (Wittmann) and bacterial lysozymes (Streisinger) from wild types and mutants confirmed the code. In 1954 Gamov tried to explain the G.c. by steric fitting of nucleotides and amino acids. In 1957, Crick suggested his Adaptor hypothesis (see), which was later confirmed experimentally. 

Secondary Structure of Ribonucleic Acid. Compared to the DNA, very little is known about the spatial arrangement of the RNA chain, i.e., about its secondary structure. For soluble RNA it is assumed that a single strand is folded back on itself giving rise in parts of the molecule to a DNA-like double helix (cf. Fig. 30 and Adaptor Hypothesis, Section 6). Base pairing and possibly double strandedness play a role also with high molecular weight ribonucleic acids, but a well-substantiated model does not exist yet. 

RNA presents mainly a sequence of sites where hydrogen bonding could occur. One would expect, therefore, that whatever went onto the template in a specific way did so by forming hydrogen bonds. It is therefore a natural hypothesis that the amino acid is carried to the template by an adaptor molecule, and that the adaptor is the part that actually fits onto the RNA. In its simplest form, one would require twenty adaptors, one for each amino acid. 

This highly innovative hypothesis soon became established as fact. The adaptor in protein synthesis is transfer RNA. The structure and reactions of these remarkable molecules will be considered in detail in Chapter 29. For the moment, it suffices to note that tRNA contains an amino acidattachment site and a template-recognition site. A tRNA molecule carries a specific amino acid in an activated form to the site of protein synthesis. The carboxyl group of this amino acid... 

This highly innovative hypothesis soon became established as fact. The adaptor in protein synthesis is transfer RNA. The structure and reactions of... 

Thus each of the models has the same basic principle—that there are certain modulator tRNA species. In the Ames and Hartman model, modulation is caused primarily by genetic change in the mRNA, and in the regulator tRNA model, by variations in the levels of regulator tRNA species, these variations stemming presumably from switching on or off of tRNA genomes. In the adaptor modification hypothesis it is postulated that modulation results from chemical or physical modifications of tRNA species. 

If the RNA world hypothesis is correct, what evidence is there that early protein synthesis mechanisms were RNA controlled First, there are three types of RNA that play major roles in translation—mRNA, which contains the genetic information tRNA, the adaptor molecule and rRNA, which comprises about 2/3 the mass of the ribosome. Mechanistically, some of the catalytic steps of protein synthesis have been mimicked using only RNA. Although several researchers have tried to demonstrate unequivocally that rRNA of modern organisms is the... 

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