Membrane synthesis in Escherichia coli is coordinated with growth via post-translational regulation

Abstract

Every cell must produce enough membrane to contain itself. However, the mechanisms by which the rate of membrane synthesis is coupled with the rate of cell growth remain unresolved. By comparing substrate and enzyme concentrations of the fatty acid and phospholipid synthesis pathways of Escherichia coli across a 3-fold range of carbon-limited growth rates, we show that the rate of membrane phospholipid synthesis during steady-state growth is determined principally through allosteric control of a single enzyme, PlsB. Due to feedback regulation of the fatty acid pathway, PlsB activity also indirectly controls synthesis of lipopolysaccharide, a major component of the outer membrane synthesized from a fatty acid synthesis intermediate. Surprisingly, concentrations of the enzyme that catalyses the committed step of lipopolysaccharide synthesis (LpxC) do not vary across steady-state growth conditions, suggesting that steady-state lipopolysaccharide synthesis is modulated primarily via indirect control by PlsB. In contrast to steady-state regulation, we find that responses to environmental perturbations are triggered directly via changes in acetyl-CoA concentrations, which enables rapid adaptation. Adaptations are further modulated by ppGpp, which regulates PlsB activity during slow growth and growth arrest. The strong reliance of the membrane synthesis pathway upon post-translational regulation ensures both reliability and responsiveness of membrane synthesis. SignificanceHow do bacteria cells grow without breaking their membranes? Although the biochemistry of fatty acid and membrane synthesis is well-known, how membrane synthesis is balanced with growth and metabolism has remained unclear. This is partly due to the many control points that have been discovered within the membrane synthesis pathways. By precisely establishing the contributions of individual pathway enzymes, our results simplify the model of membrane biogenesis in the model bacteria species Escherichia coli. Specifically, we find that allosteric control of a single enzyme, PlsB, is sufficient to balance growth with membrane synthesis and to ensure that growing E. coli produces sufficient membrane. Identifying the signals that activate and deactivate PlsB will answer the question of how membrane synthesis is synchronized with growth.