PHENYLALANYL-TRANSFER-RNA SYNTHETASE FROM THERMUS-THERMOPHILUS HAS 4 ANTIPARALLEL FOLDS OF WHICH ONLY 2 ARE CATALYTICALLY FUNCTIONAL

Phenylalanyl-tRNA synthetase from Thermus thermophilus has an alpha(2) beta(2), type quaternary structure and is one of the most complicated members of the synthetase family. Identification of PheRSTT as a member of class II aaRSs was based only on sequence alignment of the small a-subunit with other synthetases. The three-dimensional crystal structure of the catalytic and 'catalytic-like' domains at 2.9 Angstrom resolution in PheRSTT is described. The a-subunit contains an antiparallel fold which includes signature motifs 1, 2 and 3, characteristic of class II synthetases. One of the three structural domains of the beta-subunit (alpha(1)-domain) is formed by a seven-stranded antiparallel beta-sheet surrounded by alpha-helices similar to catalytic domains in SerRS, AspRS and the a-subunit of PheRSTT. The alpha beta heterodimer (alpha and alpha') exhibits essentially the same topology in the intersubunit region as in the known alpha(2) structures of class II aaRS's. The multimerization area of whole PheRSTT molecule comprises a quasi-tetrahedral four-helix bundle.