Alanine tRNA base sequence

For most of the history of mankind, unraveling the nucleotide sequence of even a quite small nucleic acid was a formidable undertaking. Following 7 years of labor, Robert Holley solved the first such structure, that for an alanine tRNA from yeast, in 1961. This molecule contains a linear chain of 76 nucleotides and includes some unusual bases, which actually help in base sequence determination. For this achievement, Holley shared the Nobel Prize in Physiology or Medicine in 1968. 

Yeast alanine tRNA (tRNA 3), the first nucleic acid to be completely sequenced (Fig. 27-11), contains 76 nucleotide residues, 10 of which have modified bases. Comparisons of tRNAs from various species have revealed many common denominators of structure (Fig. 27-12). Eight or more of the nucleotide residues have modified bases and sugars, many of which are methylated derivatives of the principal bases. Most tRNAs have a guanylate (pG) residue at the 5 end, and all have the trinucleotide sequence CCA(3 ) at the 3 end. When... 

Figure 29.3. Alanine-tRNA Sequence. The base sequence of yeast alanyl-tRNA and the deduced cloverleaf secondary structure are shown. Modified nucleosides are abbreviated as follows methylinosine (ml), dihydrouridine (UH2),...

Robert Holley first determined the base sequence of a tRNA molecule in 1965, as the culmination ul 7 years of effort, Indeed, his study of yeast alanyl-tRNA provided the first complete sequence of any nucleic acid. This adapter molecule is a single chain of 76 ribonucleotides (Figure 30.2). The 5 terminus is phosphorylated (pCi), whereas the 3 terminus has a free hydroxyl group. T he amino acid-attachment site is the 3 -hydroxyl group of the adenosine residue at the 3 terminus of the molecule. The sequence 5 - IGC-3 in the middle of the molecule is the anticodon, where I is the purine base inosine. It is complementary to 5 -GCC-3, one of the codons for alanine. 

Yeast alanine tRNA (tRNA ), the first nucleic acid to be completely sequenced (Fig. 27-11), contains 76 nucleotide residues, 10 of which have modified bases. Comparisons of tRNAs from various species have revealed many common denominators of structure (Fig. 

Figure 29-3. Structure of tRNA. Left sequence and projection of the conformation. The numbering corresponds to the phenyl alanine specific tRN A of yeast. A, Adenyl nucleoside C, cytosyl nucleoside G, guanidyl nucleoside T, thymidyl nucleoside U, uridyl nucleoside. In the case of the pseudouridyl residue the base is joined to the sugar via the C Pu is a purine nucleotide, Py is a pyrimidine nucleotide, and H is what is known as a hypermodified purine nucleotide. The other positions can be taken by any desired nucleotide, but must be complementary in the case of the hydrogen bonds signified by —. w, Wobble base. Right Spatial structure.

Figure 29-7 (A) Generalized cloverleaf diagram of all tRNA sequences except for initiator tRNAs numbered as in yeast tRNAae (Fig. 5-30). Invariant bases A, C, G, T, U, and semivariant bases Y (pyrimidine base), R (purine base), H (hypermodified purine base). The dotted regions (a, P, variable loop) contain different numbers of nucleotides in various tRNA sequences. See Rich.179 (B) L form of the yeast phenyl-alanine-specific tRNAphe. The structure is the same as that in Fig. 5-31 but has recently been redetermined at a resolution of 0.20 nm.175 The new data revealed the presence of ten bound Mg2+ ions (green circles) as well as bound spermine (green).

Since the sequence of tRNA alanine is known, a specialist can calculate the shape that the optical rotary dispersion curves would assume if the RNA is single stranded and the ordering simply results from stacking of the bases. When such calculations were compared to the actual curves, the values obtained by prediction were much smaller than the measurements. The conclusion is again that hydrogen bonds must contribute to the ordered structure of tRNA. 

They are relatively small molecules containing about 80 nucleotides. Less common purines and pyrimidines occur quite often among the base building blocks but we do not need to bother with their formulae here. In all probability certain regions of the RNA strand are paired with each other, leading to the postulated clover leaf structure. All known tRNA molecules terminate at one end with the nucleotide sequence—CCA. In 1965, Holley and his colleagues succeeded in elucidating the nucleotide sequence of a tRNA specific for the amino acid alanine. Since then the nucleotide sequence of other kinds of tRNA has also become known (Fig. 10). 

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