HUMAN CYTOPLASMIC ISOLEUCYL-TRANSFER-RNA SYNTHETASE - SELECTIVE DIVERGENCE OF THE ANTICODON-BINDING DOMAIN AND ACQUISITION OF A NEW STRUCTURAL UNIT

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.