Roles of multiple KASIII homologues of Shewanella oneidensis in initiation of fatty acid synthesis and in cerulenin resistance

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It is fully established that the condensing reaction for the initiation of fatty acid synthesis is essential for viability of many bacteria. In model bacteria such as Escherichia coli, this reaction is exclusively catalyzed by β-ketoacyl-ACP synthase (KAS) III (encoded by fabH) and the FabH loss results in a fatty acid auxotroph. However, such a notion has been under the challenge of recent findings. In an attempt to resolve the conflicting results, in this study, we examined the physiological role of multiple KASIII enzyme homologues in Shewanella oneidensis, an excellent model for researching type II fatty acid synthesis (FASII) and its regulation. We demonstrated that FabH1 and temperature-responsive FabH2 are primarily responsible for initiating synthesis of straight- and branched-chain fatty acids respectively, whereas FabH3 and OleA are dispensable. Cells lacking all these enzymes as a set are viable but carry severe defects in growth. Further analyses revealed that in the absence of KASIII either of FabB (KASI) and FabF2 (KASII) is able to support growth, suggesting that they could initiate FASII. Strikingly, KASIII enzymes and OleA together confer S. oneidensis cells resistance to cerulenin, a selective inhibitor of FabF and FabB. Along with our previous finding that S. oneidensis FabF1 and FabB are fully equivalent with respect to their physiological impacts, these results imply that physiological function promiscuity of bacterial KAS enzymes could be more extensive than previously expected.