Protein synthesis wobble base pairing

Codons that specify the same amino acid are called synonyms. Most synonyms differ only in the third base of the codon for example GUU, GUC, GUA and GUG all code for valine. During protein synthesis, each codon is recognized by a triplet of bases, called an anticodon, in a specific tRNA molecule (see Topics G10 and H2). Each base in the codon base pairs with its complementary base in the anticodon. However, the pairing of the third base of a codon is less stringent than for the first two bases (i.e. there is some wobble base-pairing ) so that in some cases a single tRNA may base-pair with more than one codon. For example, phenylalanine tRNA, which has the anticodon GAA, recognizes both of the codons UUU and UUC. The third position of the codon is therefore also called the wobble position. 

The tRNA genes (30-33) identified in cpDNA code for functional tRNAs, which form the typical clovcrlcaf structure. It is of particular imerest that tRNA (UUQ not only participates in protein synthesis, but also acts as a cofactor for the formation of 5-aminolevulinic acid, which is a common precursor in heme and chlorophyll biosynthesis. cpDNA-encoded tRNAs suffice for translation of 61 codons if normal wobble base pairing occurs in codon-anticodon recognition. Although this hypothesis is experimentally supported, transport of the nuclear-encoded tRNA into the chloroplasts cannot be excluded. Thus, the Epifagus plastid DNA contains only 17 tRNA genes, which is indirect evidence for the transport of nuclear-encoded tRNAs involved in translation. ... 

The wobble (or third) base of the codon contributes to specificity, but, because it pairs only loosely with its corresponding base in the anticodon, it permits rapid dissociation of the tRNA from its codon during protein synthesis. If all three bases of a codon engaged in strong Watson-Crick pairing with the three... 

Several hypotheses have been developed in order to interpret the paradox of a degenerate code and the specific synthesis of proteins. The wobble theory of Crick (1966 a) postulates the possibility of alternate ways of pairing between the bases of the w-RNA and -RNA. Within the framework of this theory it is assumed that the pairing for the two first positions of each codon-anticodon pair is normal, A <—>TJ, C <—>G. But in order to explain the multiple correspondence it is necessary to assume other possibilities of pairing for the third position. The postulated rela-... 

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