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Gram-negative bacteria build a multilayered cell envelope in which the peptidoglycan layer is sandwiched between an inner membrane of glycerophospholipids and an asymmetric outer membrane composed of glycerophospholipids and lipopolysaccharide (LPS). Acinetobacter baumannii , however, synthesize lipooligosaccharide (LOS), an LPS variant lacking O-antigen. Although LPS/LOS is typically essential, A. baumannii can survive without LOS, offering the opportunity to examine how the Gram-negative envelope remains stable in the absence of this major glycolipid. We previously found that the peptidoglycan biogenesis protein NlpD, an activator of peptidoglycan degradation by cell division amidases, is critical for fitness during LOS deficiency. Here, we show that NlpD is required under these conditions because a second putative amidase activator, WthA (cell wall turnover hub protein A), no longer functions in LOS-deficient cells. Mutants lacking WthA exhibited severe cell-division defects and were synthetically sick with loss of NlpD. Acinetobacter lack canonical periplasmic amidases, raising the question of which enzymes partner with NlpD and WthA. Previous work showed that overexpression of an Acinetobacter β-lactamase increased denuded peptidoglycan, a product of amidase activity. Guided by this finding, we examined the chromosomally encoded β-lactamase Oxa51 and found that its coexpression with WthA or NlpD enhanced release of amidase products, suggesting that Oxa51 participates in peptidoglycan degradation and that WthA is an amidase activator. Further, WthA influenced peptidoglycan endopeptidases and lytic transglycosylases through a network of protein interactions. Altogether, these findings identify WthA as a missing regulator in Acinetobacter peptidoglycan biogenesis and a hub that coordinates peptidoglycan turnover and cell division.
Simpson et al. (Mon,) studied this question.