Suppression of Loss of the FabH 3-Ketoacyl-ACP Synthase III by a KAS II (FabF) Lacking the Active Site Thiol in Enterococcus faecalis
Year: 2026
Type: article
Abstract: The Enterococcus faecalis fabH gene encodes the sole 3-ketoacyl-ACP synthase III of the bacterium. FabH is a conserved enzyme in bacteria and chloroplasts responsible for initiation of bacterial fatty acid synthesis. FabH enzymes generally synthesize only short chain primers (C4-C6) which are elongated to the C16 and C18 chains of bacterial phospholipids by the long chain 3-ketoacyl-ACP synthases. The long chain enzymes are classed into types I and II depending on the ability of type I enzymes to perform an early step in unsaturated acyl chain synthesis. Like the paradigm E. coli pathway, E. faecalis has two long chain 3-ketoacyl-ACP synthases of types I and II, called FabO and FabF, respectively. Deletion of the E. faecalis fabH gene (ΔfabH) results in strains requiring an unsaturated fatty acid (e.g., oleic acid) for growth and almost complete loss of fatty acid synthesis. Upon plating on a medium of limiting fatty acid content, a few large suppressor colonies appeared on a lawn of tiny colonies. The suppressor strains were found to encode mutations in the FabF long chain acyl-ACP synthase II. Strikingly, in the fabF genes of the four suppressor strains a phenylalanine codon replaced the codon of the FabF active site cysteine, C162. Since the cysteine thiol is the key active site nucleophile of 3-ketoacyl-ACP synthases, the FabF(C162F) mutant proteins lacked KAS activity. The ability of the FabF(C162F) protein to bypass loss of FabH in vivo depended on expression of a functional FabO, the other long chain KAS I. We report that suppression of lack of FabH is due to acyl-ACP primer synthesis by the FabF(C162F) protein which catalyzes transfer of the acetyl moiety of acetyl-CoA to ACP dependent on histidine H301 of the FabF(C162F) protein. The acetyl-ACP primer is utilized by FabO, to synthesize the phospholipid acyl chains required for growth of the ΔfabH strain in the absence of fatty acid supplementation.
Source: Journal of Biological Chemistry
Authors: Huijuan Dong, John E. Cronan
Language: English
Subfield: Biochemistry
Domain: Life Sciences
Open Access status: gold
Estimated APC paid: $3,430
OpenAlex ID: W7222755616