Wobble position

Clearly, the results emerging suggested that at least two nucleotides were modified, the absolute sequence position within the tRNA had yet to be established. Ching etalP showed that a Se U residue was present in the wobble position of the tRNA " from C. sticklandii. This study confirmed a notion that the modification probably affects the translation efficiency of certain transcripts, based on the level of modification by selenium. The authors speculated that the modification to seleno-tRNA (GAG) allowed for more efficient use of this tRNA species as compared to the tRNA (GAA). Even today, no definitive data exist to show that this modification alters the translation efficiency in these bacterial model systems. Nonetheless, these studies had established the chemical forms of Se U and mnm Se U, and established that they were derived from modifications to nucleotides that first required sulfur (S U and mnm S U), the mechanism by which selenium was inserted into the tRNA would not be definitively answered until many years later. 

Selenium is found to a minor extent wherever sulfur exists in nature. This includes the sulfur-containing modified bases of tRNA molecules. In addition to a small amount of nonspecific incorporation of Se into all S-containing bases there are, at least in bacteria, specific Se-containing tRNAs. In E. coli one of these is specific for lysine and one for glutamate. One of the modified bases has been identified as 5-methyl-amino-methyl-2-selenouridine.570 It is present at the first position of the anticodon, the "wobble" position.571... 

F NMR spectrum of 270 H NMR spectrum of 268 wobble position 231 Tropical macrocytic anemia 802 Tropoelastin 436 Tropomodulin 406 Tropomyosin 370,406... 

The enzyme tRNA-guanine transglycosylase (TGT) catalyzes the complete exchange of a base in tRNA [25, 26]. Upon reaction, guanine in the wobble position of tRNAs with the anticodon sequence GUN is replaced by the modified bases... 

Most amino acids in proteins are specified by more than one codon (i.e. the genetic code is degenerate). Codons that specify the same amino acid (synonyms) often differ only in the third base, the wobble position, where base-pairing with the anticodon may be less stringent than for the first two positions of the codon. 

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. 

Sturchler C, Lescure A, Keith G, Carbon P, Krol A. Base modification pattern at the wobble position of Xenopus selenocysteine tRNA(Sec). Nucleic Acids Res. 1994 22 1354-1358. 

The number of mammalian mitochondrial tRNA molecules is 22, which is less than the minimum number (32) needed to translate the universal code. This is possible because in each of the fourfold redundant sets—e.g., the four alanine codons GCU, GCC, GCA, and GCG—only one tRNA molecule (rather than two, as explained above) is used. In each set of four tRNA molecules, the base in the wobble position of the anticodon is U or a modified U (not I). It is not yet known whether this U is base-paired in the codon-anticodon interaction or manages to pair weakly with each of the four possible bases. For those codon sets that are doubly redundant—e.g., the two histidine codons CAU and CAC—the wobble base always forms, a G-U pair, as in the universal code. The structure of the human mitrochondrial tRNA molecule is also different from that of the standard tRNA molecule (except for mitochondrial tRNA UUX). (X = any nucleotide.) The most notable differences are the following ... 

Percudani, R. and Ottonello, S. (1999) Selection at the wobble position of codons read by the same tRNA in Saccharomyces cerevisiae. Mol. Biol. Evol. 16, 1752-1762. 

The interactions between the third codon and anticodon nucleotides are less stringent. In fact, nontraditional base pairs (i.e., non-Watson-Crick) often occur. For example, tRNAs containing G in the 5 (or wobble ) position of the anticodon can pair with two different codons (i.e., G can interact with either C or U). The same is true for U, which can interact with A or G. When I is in the wobble position of an anticodon, a tRNA can base pair with three different codons, because I can interact with U or A or C. 

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