FIG 7.

Summary of how an E. coli nitrogen starvation response impacts cross-feeding. The arrow thickness indicates relative flux. (A) Low NH₄⁺ excretion levels by R. palustris (Rp) limit the ability of E. coli (Ec) to obtain NH₄⁺ by diffusion across the membrane as NH₃. Low NH₄⁺ availability is sensed by E. coli through the sensor kinase NtrB, which phosphorylates the response regulator NtrC (see reference 19 for details on how nitrogen availability is sensed and transmitted). NtrC upregulates the expression of many genes involved in scavenging nitrogen, including the gene for the high-affinity NH₄⁺ transporter AmtB. Higher AmtB levels allow E. coli to acquire the small amounts of NH₄⁺ excreted by R. palustris, supporting E. coli growth and the mutualistic excretion of organic acids, which R. palustris uses as a carbon source. (B) Without NtrC, E coli AmtB levels remain low, and R. palustris has a competitive advantage in reacquiring excreted NH₄⁺. Starved for nitrogen, E. coli growth and organic acid cross-feeding slow, thereby threatening the stability of the mutualism. PTS, phosphotransferase system. (Adapted from reference 12.)