3.2.1. Primary Structure

In 1965 Holley and co-workers reported the nucleotide sequence of the first tRNA molecule . This molecule was yeast alanine tRNA (Fig. 3.1). Since then, hundreds of sequences of different tRNA from various sources have been determined. All of these structures have several common features.

The length of tRNA chains varies from 74 to 95 nucleotide residues (though in animal mitochondria it may be reduced down to 60 or even 50 nucleotides). At the 3'-end all tRNA species contain a universal trinucleotide sequence, CCAOH;« it is the terminal invariant adenosine that accepts the amino acid residue when the aminoacyl-tRNA is being formed.

The anticodon triplet is located approximately in the middle of the tRNA chain (IGC in positions 34 to 36 in Fig. •••). As a rule, the 5'-side from the anticodon contains two pyrimidine residues, whereas the 3'-side most often contains two purine residues, although the second residue on the 3'-side may be a pyrimidine, as in the case of the tRNAAla (Fig. 3.1). These seven nucleotide residues together form the so-called anticodon loop (AC loop) which interacts with the mRNA and possesses a characteristic three-dimensional structure (see below).

Approximately one-third of the way along the tRNA chain from its 3'-end there is a region common to most tRNA species; this region contains a sequence GTYC or, much less frequently, GUYC (or Gm1YYC in archaebacteria), and is flanked on both sides by purine residues. In the eukaryotic initiator tRNAfMet, this sequence is substituted for by GAYC or GAUC. This sequence is the principal conservative sequence of tRNA. In mitochondrial tRNAs, however, the corresponding sequence region varies strongly and even may be absent at all.

Some other conservative parts of the sequence in the region of nucleotide residues 8 to 25 should be mentioned. Several invariants and seminnvariants are present here: U or its thio-derivative (s4U) in position 8, G or its methyl-


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