K. Shiba et al., HUMAN CYTOPLASMIC ISOLEUCYL-TRANSFER-RNA SYNTHETASE - SELECTIVE DIVERGENCE OF THE ANTICODON-BINDING DOMAIN AND ACQUISITION OF A NEW STRUCTURAL UNIT, Proceedings of the National Academy of Sciences of the United Statesof America, 91(16), 1994, pp. 7435-7439
We show here that the class I human cytoplasmic isoleucyl-tRNA synthet
ase is an exceptionally large polypeptide (1266 aa) which, unlike its
homologues in lower eukaryotes and prokaryotes, has a third domain of
two repeats of an approximate to 90-aa sequence appended to its C-term
inal end. While extracts of Escherichia call do not aminoacylate mamma
lian tRNA with isoleucine, expression of the cloned human gene in E. c
oli results in charging of the mammalian tRNA substrate. The appended
third domain is dispensable for detection of this aminoacylation activ
ity and may be needed for assembly of a multisynthetase complex in mam
malian cells. Alignment of the sequences of the remaining two domains
shared by isoleucyl-tRNA synthetases from E. coli to human reveals a m
uch greater selective pressure on the domain needed for tRNA acceptor
helix interactions and catalysis than on the domain needed for interac
tions with the anticodon. This result may have implications for the hi
storical development of an operational RNA code for amino acids.