FabF and FadM cooperate to recycle fatty acids and rescue ∆plsX lethality in Staphylococcus aureus

Phospholipids are essential components of most cell membranes. In Staphylococcus aureus, PlsX acyltransferase is considered indispensable for initiating phospholipid synthesis, unless exogenous fatty acids (FAs) are available to bypass this requirement. We report that S. aureus can capture internal FA sources to overcome PlsX essentiality in a ∆plsX mutant via point mutations in either of two genes: fabF, which encodes the FA synthesis enzyme 3-oxoacyl-(acyl-carrier-protein) synthase II, or fadM, which encodes an understudied bifunctional acyl-CoA thioesterase and ACP binding protein. Despite growth rescue, both ∆plsX suppressors differ from the parental strain by producing phospholipids with shortened FA lengths, suggesting that both suppressors lead to premature FA release during synthesis. Additionally, both suppressors display increased sensitivity to β-lactam antibiotics. The similar behavior of both suppressors led us to show that fabF suppressors require the presence of fadM, indicative of FabF-FadM cooperation. We propose that reduced processivity of FabF suppressor variants, or greater availability of FadM for ACP binding in FadM variants, facilitates FA release from FabF-acyl-ACP intermediates. A FabF-FadM relay leading to FA release may contribute to homeostasis between FASII and phospholipid synthesis pathways.