Evidence that glutamine transaminase and omega-amidase potentially act in tandem to close the methionine salvage cycle in bacteria and plants

S-Adenosylmethionine is converted enzymatically and non-enzymatically to methylthioadenosine, which is recycled to methionine (Met) via a salvage pathway. In plants and bacteria, enzymes for all steps in this pathway are known except the last: transamination of alpha-ketomethylthiobutyrate to give Met. In mammals, glutamine transaminase K (GTK) and omega-amidase (omega-Am) are thought to act in tandem to execute this step, with GTK forming alpha-ketoglutaramate, which omega-Am hydrolyzes. Comparative genomics indicated that GTK and omega-Am could function likewise in plants and bacteria because genes. encoding GTK and omega-Am homologs (i) co-express with the Met salvage gene 5-methylthioribose kinase in Arabidopsis, and (ii) cluster on the chromosome with each other and with Met salvage genes in diverse bacteria. Consistent with this possibility, tomato, maize, and Bacillus subtilis GTK and omega-Am homologs had the predicted activities: GTK was specific for glutamine as amino donor and strongly preferred alpha-ketomethylthiobutyrate as amino acceptor, and omega-Am strongly preferred alpha-ketoglutaramate. Also consistent with this possibility, plant GTK and omega-Am were localized to the cytosol, where the Met salvage pathway resides, as well as to organelles. This multiple targeting was shown to result from use of alternative start codons. In B. subtilis, ablating GTK or omega-Am had a modest but significant inhibitory effect on growth on 5-methylthioribose as sole sulfur source. Collectively, these data indicate that while GTK, coupled with omega-Am, is positioned to support significant Met salvage flux in plants and bacteria, it can probably be replaced by other aminotransferases. (C) 2014 Elsevier Ltd. All rights reserved.