Fig. 1: Homeoviscous adaptation and membrane metabolism in E. coli.

A E. coli maintains membrane fluidity by titrating the fraction of membrane phospholipids bearing unsaturated fatty acids. B The E. coli fatty acid synthesis pathway splits into saturated and unsaturated branches at the enoyl-acyl-ACP intermediate C10:1(2E) ACP. Reduction by the enoyl-acyl-ACP reductase FabI initiates synthesis of saturated fatty acid thioesters (primarily C16:0 ACP). Alternatively, reversible isomerization of C10:1(2E) ACP by the bifunctional hydroxyl-acyl-ACP dehydratase/enoyl-acyl-ACP isomerase FabA generates C10:1(3Z) ACP, a substrate for the β-keto-acyl-ACP synthase FabB. As C10:1(2E) and C10:1(3Z) ACP are reversibly interconverted by FabA, FabI and FabB indirectly compete for a common pool of substrates. The FabB reaction initiates synthesis of the unsaturated fatty acid thioester C16:1 ACP. A fraction of the C16:1 ACP pool is elongated by the β-keto-acyl synthase enzyme FabF, ultimately generating C18:1 ACP. Phospholipids are synthesised by PlsB and PlsC, which transfer acyl groups to glycerol-3-phosphate (G3P) to produce phosphatidic acid (PA). The PA headgroup is further modified to yield membrane phospholipids phosphatidylglycerol (PG) and phosphatidylethanolamine (PE). Complete pathways are depicted in Supplemental Fig. 1. C C16:0 and C18:1 ACP compete for binding to the transcriptional regulator FabR. The FabR-C18:1 ACP complex represses fabB transcription, while FabR binding to C16:0 ACP relieves repression. FabA expression is primarily controlled in response to fatty acyl-CoA generated from exogenous fatty acids by the transcriptional regulator FadR³³. D LCMS quantification of long-chain acyl-ACP, PA, PG, and PE. Symbols are LCMS counts measured from one technical replicate (of 3 total) from 2 independent cultures (cultures distinguished by solid black and open red symbols). Lines indicate the average obtained for the cultures (solid black and dashed red lines). Values are normalised to fraction of total LCMS counts per unit biomass (determined by optical density (OD)), which approximates relative concentrations. Areas represent total LCMS counts for phospholipid species bearing acyl chains at either the sn-1 or sn-2 positions. E Relative abundance of FabA and FabB enzymes in cultures maintained at 5 temperatures. 3 technical measurements (symbols) and their average (line) are depicted for 2 independent cultures (distinguished by colour) at each temperature. F Phospholipid compositions of strains overexpressing YFP (control), FabA, and FabB enzymes. Bars depict average of 3 technical measurements (depicted by symbols) from one culture each. Source data are provided as a Source Data file.