title: Need-based activation of ammonium uptake in Escherichia coli.
abstract: The efficient sequestration of nutrients is vital for the growth and survival of microorganisms. Some nutrients, such as CO2 and NH3, are readily diffusible across the cell membrane. The large membrane permeability of these nutrients obviates the need of transporters when the ambient level is high. When the ambient level is low, however, maintaining a high intracellular nutrient level against passive back diffusion is both challenging and costly. Here, we study the delicate management of ammonium (NH4+/NH3) sequestration by E. coli cells using microfluidic chemostats. We find that as the ambient ammonium concentration is reduced, E. coli cells first maximize their ability to assimilate the gaseous NH3 diffusing into the cytoplasm and then abruptly activate ammonium transport. The onset of transport varies under different growth conditions, but always occurring just as needed to maintain growth. Quantitative modeling of known interactions reveals an integral feedback mechanism by which this need-based uptake strategy is implemented. This novel strategy ensures that the expensive cost of upholding the internal ammonium concentration against back diffusion is kept at a minimum.
authors.affiliation: Department of Physics, University of California at San Diego, La Jolla, CA, USA.
authors.forename: Minsu
authors.initials: M
authors.lastname: Kim
authors.forename: Zhongge
authors.initials: Z
authors.lastname: Zhang
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Okano
authors.forename: Dalai
authors.initials: D
authors.lastname: Yan
authors.forename: Alexander
authors.initials: A
authors.lastname: Groisman
authors.forename: Terence
authors.initials: T
authors.lastname: Hwa
mesh_terms: D027682:Cation Transport Proteins / Q000378:metabolism; D004926:Escherichia coli / Q000166:cytology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000378:metabolism; D025461:Feedback, Physiological / Q000187:drug effects; D015964:Gene Expression Regulation, Bacterial / Q000187:drug effects; D005974:Glutamate-Ammonia Ligase / Q000378:metabolism; D005973:Glutamine / Q000494:pharmacology; D008954:Models, Biological; D009584:Nitrogen / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism* / Q000494:pharmacology; D015854:Up-Regulation / Q000187:drug effects
chemical_list: C111096:AmtB protein, E coli; D027682:Cation Transport Proteins; D029968:Escherichia coli Proteins; D000644:Quaternary Ammonium Compounds; D005973:Glutamine; D005974:Glutamate-Ammonia Ligase; D009584:Nitrogen