Metabolic rerouting by gain-of-function mutations overcomes plsX essentiality in Staphylococcus aureus
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Phospholipids are essential components of most bacterial membranes. The PlsX acyltransferase initiates phospholipid synthesis by reversibly converting acyl-ACP (ACP, acyl carrier protein) to acyl-PO 4 , the first phospholipid precursor. PlsX is dispensable in some bacteria, as alternative functions assure acyl-PO 4 production. In contrast, PlsX is considered indispensable in Staphylococcus aureus , unless exogenous fatty acids (FA) are provided. Here we report that gain-of-function suppressors of Δ pls X enable S. aureus growth without FA addition. The suppressors map to either the FA synthesis (FASII) enzyme FabF, or to a putative acyl-CoA thioesterase/ACP binding protein, designated FadM. The Δ plsX suppressors alleviate accumulation of long chain acyl-ACP, and produce shorter chain membrane FAs than the parental strain. Unlike wild type S. aureus , which can adapt to FASII inhibitors, Δ plsX suppressors remain fully sensitive, indicating that PlsX reverse activity is not compensated. Importantly, an interdependency between FabF and FadM functions shown here suggests a more general role for FadM in facilitating FA release from FabF-FA-ACP intermediates. In summary, S. aureus fabF and fadM mutations promote enzymatic flexibility that overcomes Δ pls X growth arrest by restoring phospholipid synthesis.
Importance
Phospholipids are common and vital cell membrane components. Many bacteria initiate phospholipid synthesis using an enzyme called PlsX; some of them also have a backup system to ensure phospholipid synthesis and survival even if PlsX is disabled. However Staphylococcus aureus , a major pathogen, reportedly lacked a PlsX backup system. Here we used a genetic selection approach to show that S. aureus generates “gain-of-function” mutations, allowing bacteria to initiate phospholipid synthesis when PlsX is inactive. Bacteria survive thanks to a mutation in FabF, a fatty acid biosynthesis enzyme, or in FadM, a little-studied protein; our data indicate that FabF and FadM cooperate to compensate a PlsX defect. The outcome with either mutation is that bacteria produce and release fatty acids to patch into phospholipid synthesis. This study illustrates how new enzymatic circuitry is created by mutation to rescue bacteria from a potentially lethal membrane phospholipid synthesis defect.
