Loss of YhcB results in overactive fatty acid biosynthesis

Loss of the Escherichia coli inner membrane protein YhcB results in pleomorphic cell morphology and clear growth defects. Prior work suggested that YhcB was directly involved in cell division or peptidoglycan assembly. We found that loss of YhcB is detrimental in genetic backgrounds in which lipopolysaccharide (LPS) or glycerophospholipid (GPL) synthesis is altered. The growth defect of ΔyhcB could be rescued through inactivation of the Mla pathway, a system responsible for the retrograde transport of GPLs that are mislocalized to the outer leaflet of the outer membrane. Interestingly, this rescue was dependent upon the outer membrane phospholipase PldA that cleaves GPLs at the bacterial surface. Since the freed fatty acids resulting from PldA activity serve as a signal to the cell to increase LPS synthesis, this result suggested that outer membrane lipids are imbalanced in ΔyhcB. Mutations that arose in ΔyhcB populations during two independent suppressor screens were in genes encoding subunits of the acetyl coenzyme A carboxylase complex, which initiates fatty acid biosynthesis (FAB). These mutations fully restored cell morphology and reduced GPL levels, which were increased compared to wild-type bacteria. Growth of ΔyhcB with the FAB-targeting antibiotic cerulenin also increased cellular fitness. Furthermore, genetic manipulation of FAB and lipid biosynthesis showed that decreasing FAB rescued ΔyhcB filamentation, whereas increasing LPS alone could not. Altogether, these results suggest that YhcB may play a pivotal role in regulating FAB and, in turn, impact cell envelope assembly and cell division.IMPORTANCESynthesis of the Gram-negative cell envelope is a dynamic and complex process that entails careful coordination of many biosynthetic pathways. The inner and outer membranes are composed of molecules that are energy intensive to synthesize, and, accordingly, these synthetic pathways are under tight regulation. The robust nature of the Gram-negative outer membrane renders it naturally impermeable to many antibiotics and therefore a target of interest for antimicrobial design. Our data indicate that when the inner membrane protein YhcB is absent in Escherichia coli, the pathway for generating fatty acid substrates needed for all membrane lipid synthesis is dysregulated which leads to increased membrane material. These findings suggest a potentially novel regulatory mechanism for controlling the rate of fatty acid biosynthesis.