Glutamine synthetase II in Rhizobium: reexamination of the proposed horizontal transfer of DNA from eukaryotes to prokaryotes

We have determined the DNA sequence of a Rhizobium meliloti gene that encodes glutamine synthetase II (GSII). The deduced amino acid sequence was compared to that of Bradyrhizobium japonicum GSII and those of various plant and mammalian glutamine synthetases (GS) in order to evaluate a proposal that the gene for this enzyme was recently transferred from plants to their symbiotic bacteria. There is 83.6% identity between the R. meliloti and B. japonicum proteins. The bacterial GSII proteins average 42.5% identity with the plant GS proteins and 41.8% identity with their mammalian counterparts. The plant proteins average 53.7% identity with the mammalian proteins. Thus, the GS proteins are highly conserved and the divergence of these proteins is proportional to the phylogenetic divergence of the organisms from which the sequences were determined. No transfer of genes across large taxonomic gaps is needed to explain the presence of GSII in these bacteria.