AN ALTERNATIVE SPLICING MODIFIES THE C-TERMINAL END OF TRYPTOPHANYL-TRANSFER-RNA SYNTHETASE IN MURINE EMBRYONIC STEM CELLS

The cloning of murine tryptophanyl-tRNA synthetase revealed the existe nce of at least three messenger RNAs able to code for this enzyme. In most of the tissues tested, two major mRNA species were detected. They are produced by alternative polyadenylation and they share the same o pen reading frame. The deduced peptidic sequence is highly homologous to bovine and human tryptophanyl-tRNA synthetases. In embryonic stem c ells, a third type of mRNA was characterized. Surprisingly, this mRNA contains, at the C terminus of the open reading frame, a sequence codi ng for six additional amino acids. Southern blot and polymerase chain reaction analysis showed that the two open reading frames are encoded by the same gene. Thus, alternative splicing may generate two tryptoph anyl-tRNA synthetase isoforms. This phenomenon is the first reported c ase for an aminoacyl-tRNA synthetase mRNA with two open reading frame isoforms. Moreover, to our knowledge, this is the first time that a pe ptidic addition to the COOH terminus of a protein, by mRNA alternative splicing, is described. The extra hexapeptide, Cys-Phe-Cys-Phe-Asp-Th r-COOH, resembles the consensus sequence found in C termini of Ras pro teins.