  Ammonium assimilation in E. coli is regulated by two paralogous proteins
(GlnB and GlnK), which orchestrate interactions with regulators of gene
expression, transport proteins and metabolic pathways. Yet how they conjointly
modulate the activity of glutamine synthetase (GS), the key enzyme for nitrogen
assimilation, is poorly understood. We combine experiments and theory to study
the dynamic roles of GlnB and GlnK during nitrogen starvation and upshift. We
measure time-resolved in vivo concentrations of metabolites, total and
post-translationally modified proteins, and develop a concise biochemical model
of GlnB and GlnK that incorporates competition for active and allosteric sites,
as well as functional sequestration of GlnK. The model predicts the responses
of GS, GlnB and GlnK under time-varying external ammonium level in the wild
type and two genetic knock-outs. Our results show that GlnK is tightly
regulated under nitrogen-rich conditions, yet it is expressed during ammonium
run-out and starvation. This suggests a role for GlnK as a buffer of nitrogen
shock after starvation, and provides a further functional link between nitrogen
and carbon metabolisms.
