WIDESPREAD USE OF THE GLU-TRNA(GLN) TRANSAMIDATION PATHWAY AMONG BACTERIA - A MEMBER OF THE ALPHA-PURPLE BACTERIA LACKS GLUTAMINYL-TRANSFER-RNA SYNTHETASE

The expression of the Rhizobium meliloti glutamyl-tRNA synthetase gene in Escherichia coli under the control of a trc promoter results in a toxic effect upon isopropyl-beta-D-thiogalactopyranoside induction, wh ich is probably caused by a misacylation activity. To further investig ate this unexpected result, we looked at the pathway of Gln-tRNA(Gln) formation in R, meliloti. No glutaminyl-tRNA synthetase activity has b een found in R. meliloti crude extract, but we detected a specific ami notransferase activity that changes Glu-tRNA(Gln) to Gln-tRNA(Gln). Ou r results show that R. meliloti, a member of the alpha-subdivision of the purple bacteria, is the first Gram-negative bacteria reported to u se a transamidation pathway for Gln-tRNA(Gln) synthesis, A phylogeneti c analysis of the contemporary glutamyl-tRNA synthetase and glutaminyl -tRNA synthetase amino acid sequences reveals that a close evolutionar y relationship exists between R. meliloti and yeast mitochondrial glut amyl-tRNA synthetases, which is consistent with an origin of mitochond ria in the alpha-subdivision of Gram-negative purple bacteria. A 256-a mino acid open reading frame closely related to bacterial glutamyl-tRN A synthetases, which probably originates from a glutamyl-tRNA syntheta se gene duplication, was found in the 4-min region of the E. coli chro mosome. We suggest that this open reading frame is a relic of an ancie nt transamidation pathway that occurred in an E. coli ancestor before the horizontal transfer of a eukaryotic glutaminyl-tRNA synthetase (La mour, V., Quevillon, S., Diriong, S., N'Guyen, V. C., Lipinski, M., an d Mirande, M. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 8670-8674) an d that it favored its stable acquisition, From these observations, a r evisited model for the evolution of the contemporary glutamyl-tRNA syn thetases and glutaminyl-tRNA synthetases that differs from the general ly accepted model for the evolution of aminoacyl-tRNA synthetases is p roposed.