Alternate fatty acid synthesis initiation in Escherichia coli

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Fatty acid synthesis is important in both prokaryotes and eukaryotes for formation of functional membranes and dysregulation of the pathway has implications for cell growth and survival. In bacteria, fatty acid production is tightly regulated by the Type II fatty acid synthesis (FASII) pathway. FabH (3‐ketoacyl acyl carrier protein (ACP) synthase III) initiates fatty acid synthesis by condensing acetyl coenzyme A (acetyl‐CoA) with malonyl‐ACP in Escherichia coli. FabH was long thought to be the sole enzyme responsible for initiation of FASII and therefore essential for survival. The observation that fabH deletion strains are viable suggested the existence of an alternate initiation pathway. The yiiD gene was recently identified as essential for initiation of fatty acid synthesis in the absence of FabH in E. coli. The biochemical activity of YiiD is not known.YiiD has an N‐terminal GCN5‐related N‐acetyltransferase (GNAT) domain and a C‐terminal hotdog domain that is annotated as a thioesterase. The full‐length protein, as well as the N‐terminal and C‐terminal domains were purified using an E. coli expression system. Full length YiiD and the C‐terminal hotdog domain are dimers and the N‐terminal GNAT domain is a monomer.YiiD lacks condensing enzyme activity, suggesting that the initiation of FASII by YiiD is occurring through an alternate mechanism. We hypothesized that YiiD is initiating FASII by a transacylase reaction. FabH has acetyl‐CoA:ACP transacetylase activity, however protein extracts from an E. coli fabH deletion strain retain transacetylase activity. Purified YiiD has acetyl‐CoA:ACP transacetylase activity. The N‐terminal GNAT domain was inactive, whereas the C‐terminal domain was 100‐fold more active than intact YiiD. This suggests that the C‐terminal domain has the transacetylase active site and that the N‐terminal domain negatively regulates this activity. These data are consistent with YiiD forming acetyl‐ACP that is used by the elongation condensing enzymes (FabB or FabF) to initiate FASII thus bypassing FabH.Support or Funding InformationNational Institutes of Health Grant GM034496, Cancer Center (CORE) Support Grant CA21765, American and Lebanese Syrian Associated Charities (ALSAC)Proposed model for alternate initiation of fatty acid synthesis by YiiD in E. coliFigure 1