The Bacillus subtilis glutamyl-tRNA synthetase (GluRS), encoded by the gltX gene, aminoacylates its homologous tRNA(Glu) and tRNA(Gln) with glutamate. This gene was cloned with its sigma A promoter and a downstream region including a rho-independent terminator in the shuttle vector pRB394 for Escherichia coli and B. subtilis. Transformation of B. subtilis with this recombinant plasmid (pMP411) led to a 30-fold increase of glutamyl-tRNA synthetase specific activity in crude extracts. Transformation of E. coli with this plasmid gave no recombinants, but transformation with plasmids bearing an altered gltX was successful. These results indicate that the presence of B. subtilis glutamyl-tRNA synthetase is lethal for E. coli, probably because this enzyme glutamylates tRNA1(Gln) in vivo as it does in vitro.