Exogenous fatty acids inhibit fatty acid synthesis through competition between endogenously- and exogenously-generated substrates for phospholipid synthesis in Escherichia coli
Exogenous fatty acids are directly incorporated into bacterial membranes, heavily influencing bacterial ecology and antibiotic susceptibility. We use liquid chromatography/mass spectrometry to characterize how exogenous fatty acids impact the Escherichia coli fatty acid synthesis pathway. We find that acyl-CoA synthesized from exogenous fatty acids rapidly increases long-chain acyl-ACP levels while depleting malonyl-ACP, indicating inhibition of fatty acid synthesis. Contrary to previous assumptions, acyl-CoA does not inhibit FabI in vivo; instead, substrate competition between acyl-CoA and acyl-ACP for phospholipid synthesis enzymes causes long-chain acyl-ACP to accumulate, inhibiting fatty acid synthesis initiation. Furthermore, changes in the acyl-ACP pool driven by acyl-CoA amplify the effects of exogenous fatty acids on the balance between saturated and unsaturated membrane lipids. Transcriptional regulation rebalances saturated and unsaturated acyl-ACP by adjusting FabA and FabB expression. Remarkably, all other fatty acid synthesis enzymes remain at stable levels, maintaining a fixed synthesis capacity despite the availability of exogenous fatty acids. Since all bacterial pathways for exogenous fatty acid incorporation characterized so far converge with endogenous synthesis pathways in a common substrate pool, we propose that the substrate competition-triggered feedback mechanism identified here is ubiquitous across bacterial species.