Data and COPASI codes for van den Berg et al., 2024

Exogenous fatty acids are directly incorporated into bacterial membranes, heavily influencing bacterial ecology and antibiotic susceptibility. We use liquid chromatography/mass spectroscopy 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 PlsB and PlsC 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, allm other fatty acid synthesis enzymes remain at stable levels, maintaining a fixed synthesis capacity despite the availability of exogenous fatty acids. Since all 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-based feedback mechanism identified here is ubiquitous across bacterial species.