A potential role for acyl-phosphate in the coordination of phospholipid and lipopolysaccharide synthesis in Escherichia coli

Abstract

The envelope of Gram-negative bacteria like Escherichia coli is multilayered with two membranes sandwiching a peptidoglycan cell wall. The inner membrane is a typical phospholipid bilayer whereas the outer membrane is asymmetric with phospholipids in the inner leaflet and lipopolysaccharide (LPS) in the outer leaflet. Over the last several decades, most proteins essential for Gram-negative cell surface assembly have been characterized. However, relatively little is known about how the synthesis of different envelope layers is coordinated to promote uniform surface growth. We recently discovered that inactivation of the peptidoglycan synthesis machinery responsible for cell elongation causes defects in both peptidoglycan and LPS synthesis in E. coli. Here, we report the results of a transposon sequencing-based screen for suppressors that rescue the growth phenotype caused by an impaired cell elongation system. Inactivation of the phospholipid synthesis enzyme PlsX was found to partially suppress the growth and morphology defects of a cell elongation hypomorph. Deletion of plsX also conferred increased resistance to CHIR-090, an inhibitor of the committed step of LPS synthesis catalyzed by LpxC, suggesting that loss of PlsX function stimulates LPS synthesis. Evidence is presented that increased CHIR-090 resistance is not mediated by changes in the activity of the well-characterized proteolytic system (YejM-LapB-FtsH) controlling LpxC turnover. Rather, our results are consistent with a model in which the phospholipid precursor acyl-phosphate produced by PlsX serves as a direct inhibitor of LpxC to lower the rate of LPS synthesis when phospholipid synthesis capacity is reduced.