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 α-ketomethylthiobutyrate to give Met. In mammals, glutamine transaminase K (GTK) and ω-amidase (ω-Am) are thought to act in tandem to execute this step, with GTK forming α-ketoglutaramate, which ω-Am hydrolyzes. Comparative genomics indicated that GTK and ω-Am could function likewise in plants and bacteria because genes encoding GTK and ω-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 ω-Am homologs had the predicted activities: GTK was specific for glutamine as amino donor and strongly preferred α-ketomethylthiobutyrate as amino acceptor, and ω-Am strongly preferred α-ketoglutaramate. Also consistent with this possibility, plant GTK and ω-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 ω-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 ω-Am, is positioned to support significant Met salvage flux in plants and bacteria, it can probably be replaced by other aminotransferases.
Horticultural Sciences Department, University of Florida, Gainesville, FL 32611, USA. Electronic address: kellens@ufl.edu.
Kenneth W
KW
Ellens
Department of Molecular and Cellular Biology, University of Guelph, Guelph, ON N1G 2W1, Canada.
Lynn G L
LG
Richardson
Horticultural Sciences Department, University of Florida, Gainesville, FL 32611, USA.
Océane
O
Frelin
Plant Molecular and Cellular Biology Program, University of Florida, Gainesville, FL 32611, USA.
Joseph
J
Collins
Plant Molecular and Cellular Biology Program, University of Florida, Gainesville, FL 32611, USA.
Cintia Leite
CL
Ribeiro
Plant Molecular and Cellular Biology Program, University of Florida, Gainesville, FL 32611, USA.
Yih-Feng
YF
Hsieh
Department of Molecular and Cellular Biology, University of Guelph, Guelph, ON N1G 2W1, Canada.
Robert T
RT
Mullen
Horticultural Sciences Department, University of Florida, Gainesville, FL 32611, USA.
Andrew D
AD
Hanson
D000581:Amidohydrolases / Q000378:metabolism*; D017360:Arabidopsis / Q000737:chemistry / Q000201:enzymology / Q000235:genetics / Q000378:metabolism; D001412:Bacillus subtilis / Q000378:metabolism*; D007656:Ketoglutaric Acids / Q000378:metabolism; D008190:Lyases / Q000378:metabolism*; D018551:Solanum lycopersicum / Q000378:metabolism; D008715:Methionine / Q000378:metabolism*; D006109:Poaceae / Q000378:metabolism*; D012436:S-Adenosylmethionine / Q000378:metabolism*; D019657:Solanaceae / Q000378:metabolism*; D000637:Transaminases / Q000378:metabolism*; D003313:Zea mays / Q000737:chemistry / Q000201:enzymology / Q000235:genetics / Q000378:metabolism
D007656:Ketoglutaric Acids; C008230:alpha-ketoglutaramate; D012436:S-Adenosylmethionine; D008715:Methionine; D000637:Transaminases; C020656:glutamine-pyruvate aminotransferase; C427605:glutamine - phenylpyruvate transaminase; D000581:Amidohydrolases; C047679:omega-amidase; D008190:Lyases
Comparative genomics; Glutamine transaminase; Maize; Methionine salvage pathway; Nicotiana tabacum cv. Bright Yellow-2 Solanaceae; Poaceae; Solanaceae; Solanum lycopersicum cv. Floridade; Tobacco; Tomato; Yang cycle; Zea mays; ω-Amidase