All tRNAs have the same general shape, resembling the letter ‘L’, and all have a CCA sequence at the
3 terminus that serves as amino-acid acceptor (Figure 7.6). Here, the ribose of the 3-terminal A becomes
linked to the amino acid viaan ester linkage. The synthetases either link the amino acid onto the tRNA at
the ribose 2-hydroxyl group (Class I) or at the 3-hydroxyl group (Class II).
The classes of synthetase have different structures and ways of recognizing their cognate tRNA. In all
cases, a single domain binds the amino acid, ATP and the acceptor 3-terminal A residue. The Class I group
uses a variant of the ‘Rossmann fold’– a common dinucleotide-binding domain composed typically of six
parallel -strands. In the synthetases, only five parallel strands are found. Class II synthetases use instead
an anti-parallel six-stranded -sheet.
The first crystal structure of a tRNA-synthetase complex to be solved, which was also the first protein–
RNA complex structure to be studied, was of E. coliglutaminyl-tRNA synthetase with its cognate tRNA
Protein–Nucleic Acid Interactions 419
Figure 10.22 The complex of three of the nine zinc fingers for transcription factor TFIIIA with a portion of 5S RNA
of the ribosome (PDB: 1VN6)