Abstract
The bacterial cell wall separates the cell from its surrounding and protects it from environmental stressors. Its integrity is maintained by a highly regulated process of cell wall biosynthesis. The membrane-located lipid II cycle provides cell wall building blocks that are assembled inside the cytoplasm to the outside for incorporation. Its carrier molecule, undecaprenyl phosphate (UP), is then recycled by dephosphorylation from undecaprenyl pyrophosphate (UPP). In Bacillus subtilis, this indispensable reaction is catalyzed by the UPP phosphatases BcrC and UppP. Here, we study the physiological function of both phosphatases with respect to morphology, cell wall homeostasis and the resulting cell envelope stress response (CESR). We demonstrate that uppP and bcrC represent a synthetic lethal gene pair, which encodes an essential physiological function. Accordingly, cell growth and morphology were severely impaired during exponential growth if the overall UPP phosphatase level was limiting. UppP, but not BcrC, was crucial for normal sporulation. Expression of bcrC, but not uppP, was upregulated in the presence of cell envelope stress conditions caused by bacitracin if UPP phosphatase levels were limited. This homeostatic feedback renders BcrC more important during growth than UppP, particularly in defense against cell envelope stress.
Highlights
The bacterial cell wall is an essential structure that gives the cell its shape and counteracts the turgor pressure
We provide the first evidence that DgkA is involved in undecaprenyl pyrophosphate (UPP) homeostasis: While a lack of this predicted undecaprenol kinase did not result in an observable deficiency, a role in undecaprenyl phosphate (UP) turnover is indicated by an increased activity of PbcrC in a dgkA mutant in stationary phase
We demonstrated that uppP and bcrC constitute a synthetic lethal gene pair – a result that is in perfect agreement with an independent study performed in parallel using CRISPRdCas9 knock-downs to study the effect of UPP phosphatase levels in B. subtilis (Zhao et al, 2016)
Summary
The bacterial cell wall is an essential structure that gives the cell its shape and counteracts the turgor pressure. ΣM controls approximately 60 genes involved in cell wall synthesis, shape determination, detoxification and DNA damage response (Eiamphungporn and Helmann, 2008) It is activated by multiple triggers, including antibiotics, high salt, heat stress, and acidic pH (Thackray and Moir, 2003). This lead to the hypothesis that changes in UP and UPP levels can be sufficient to create CES (Radeck et al, 2016b) Due to their crucial role in the lipid II cycle, we hypothesize that impaired UPP phosphatase activity leads to a limitation in cell wall synthesis, which in turn should increase the CESR. Our data provides the first insight into the fine-tuning of UP homeostasis that adjusts the Lipid II cycle, and cell wall biosynthesis, in response to growth rates and envelope stress levels
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