title: Robust control of nitrogen assimilation by a bifunctional enzyme in E. coli.
abstract: Bacteria regulate the assimilation of multiple nutrients to enable growth. How is balanced utilization achieved, despite fluctuations in the concentrations of the enzymes that make up the regulatory circuitry? Here we address this question by studying the nitrogen system of E. coli. A mechanism based on the avidity of a bifunctional enzyme, adenylyltransferase (AT/AR), to its multimeric substrate, glutamine synthetase, is proposed to maintain a robust ratio between two key metabolites, glutamine and α-ketoglutarate. This ratio is predicted to be insensitive to variations in protein levels of the core circuit and to the rate of nitrogen utilization. We find using mass spectrometry that the metabolite ratio is robust to variations in protein levels and that this robustness depends on the bifunctional enzyme. Moreover, robustness carries through to the bacteria growth rate. Interrupting avidity by adding a monofunctional AT/AR mutant to the native system abolishes robustness, as predicted by the proposed mechanism.
authors.affiliation: Department of Molecular Cell Biology, Weizmann Institute of Science, Rehovot 76100, Israel.
authors.forename: Yuval
authors.initials: Y
authors.lastname: Hart
authors.forename: Daniel
authors.initials: D
authors.lastname: Madar
authors.forename: Jie
authors.initials: J
authors.lastname: Yuan
authors.forename: Anat
authors.initials: A
authors.lastname: Bren
authors.forename: Avraham E
authors.initials: AE
authors.lastname: Mayo
authors.forename: Joshua D
authors.initials: JD
authors.lastname: Rabinowitz
authors.forename: Uri
authors.initials: U
authors.lastname: Alon
mesh_terms: D002244:Carbon / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology* / Q000254:growth & development; D029968:Escherichia coli Proteins / Q000378:metabolism / Q000502:physiology*; D005974:Glutamate-Ammonia Ligase / Q000378:metabolism; D013058:Mass Spectrometry; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism*; D009713:Nucleotidyltransferases / Q000502:physiology*; D055503:Protein Multimerization
chemical_list: D029968:Escherichia coli Proteins; D002244:Carbon; D009713:Nucleotidyltransferases; C020658:glutamine-synthetase adenylyltransferase; D005974:Glutamate-Ammonia Ligase; D009584:Nitrogen