Characterizing the Staphylococcus aureus fatty acid degradation operon

Staphylococcus aureus can supplement its endogenous fatty acid synthesis pathway (FASII) with exogenous fatty acids it acquires from the environment through the fatty acid kinase (Fak) complex. Although S. aureus has been thought to not degrade fatty acids, it does possess a potential fadXDEBA locus that contains all the genes necessary for β-oxidation. Using mRNA analysis, we determined that the fadXDEBA operon can be found on one polycistronic mRNA. Moreover, we identified the fadX promoter and a putative binding site within this region that is consistent with negative regulation by the metabolism-responsive regulator, Carbon Catabolite Protein A (CcpA). Indeed, in the absence of glucose or CcpA, we saw the fadXDEBA operon was derepressed. S. aureus is annotated to lack the crotonase domain of FadB; however, new analysis indicates it is present. To test the functionality of the S. aureus FadB, we performed complementation assays with E. coli fad mutants using minimal media supplemented with single fatty acids. We were able to restore the growth of E. coli fad mutants when providing safadBA genes on a plasmid and demonstrate that the SaFadB crotonase domain is required for complementation. Together, these data demonstrate the SaFadBA proteins are functional within a well-characterized fatty acid degradation system, and the S. aureus fadXDEBA operon is under strong catabolite repression.

Staphylococcus aureus has long been thought to lack a functional fatty acid degradation (Fad) pathway based on limited studies. Pathway analysis suggested the S. aureus FadB protein lacks a crotonase domain, which is essential for Fad activity. This study demonstrates that S. aureus FadB possesses a crotonase domain that has eluded identification likely due to the orientation of its two enzymatic domains. In addition, we show that the Fad pathway is under strong catabolite repression under standard laboratory conditions, which may have also contributed to its lack of detected activity. A new model of fatty acid metabolism is emerging in S. aureus that changes the understanding of how this bacterium synthesizes and metabolizes fatty acids.