HUMAN GLYCYL-TRANSFER-RNA SYNTHETASE - WIDE DIVERGENCE OF PRIMARY STRUCTURE FROM BACTERIAL COUNTERPART AND SPECIES-SPECIFIC AMINOACYLATION

Several class I and class II human tRNA synthetases are clearly relate d to their bacterial counterparts. We report here the cloning, cDNA se quence, deduced primary structure, and expression in bacteria of a cla ss II human glycyl-tRNA synthetase. While the human sequence aligns we ll with a Bombyx mori and a Saccharomyces cerevisiae sequence for glyc yl-tRNA synthetase, particularly in the region of the class II-definin g sequence motifs, it diverges widely from that of the Escherichia col i enzyme. The divergence is so great that from the sequences alone we cannot conclude that the human and E. coli proteins are descended from homologous genes. Moreover, even though the human and E. coli class I I alanyl-tRNA synthetases cross-acylate their respective tRNAs, aminoa cylations by the recombinant human and E. coli glycyl-tRNA synthetases are restricted to their homologous tRNAs, The species-specific aminoa cylations correlate with a nucleotide sequence difference at a locatio n in the acceptor stem that is known to be critical for aminoacylation s by the E. coli enzyme. Thus, glycyl-tRNA synthetase may have followe d a path of historical development different in at least some respects from that of several other tRNA synthetases.