# Regulated Assembly of LPS, Its Structural Alterations and Cellular Response to LPS Defects
## 4 Regulation of Synthesis of UDP-GlcNAc-Precursor for LPS Biosynthesis  
UDP-\(N\)-acetyl-d-glucosamine (UDP-GlcNAc) is a common metabolic precursor for LPS and peptidoglycan synthesis. Thus, regulation of biosynthesis of UDP-GlcNAc serves as an essential branch point in controlling and coupling the synthesis of major essential constituents of the cell envelope [13]. GlmS catalyzes the synthesis of glucosamine-6-phosphate (GlcN6P) from fructose-6-phosphate and glutamine, which constitutes the first committed step in the synthesis of UDP-GlcNAc. The amount of _glmS_ transcript is regulated by a feedback mechanism in response to the GlcN6P level using homologous GlmZ and GlmY sRNAs [26; 27]. These sRNAs act in a hierarchical manner to activate the _glmS_ expression. Under GlcN6P limiting conditions, the GlmY sRNA accumulates and sequesters RNase adaptor protein RapZ, preventing GlmZ processing [26; 27]. The GlmZ sRNA facilitates translation of the _glmS_ mRNA through an anti-antisense mechanism and prevents the formation of an inhibitory structure that occludes the ribosome-binding site of _glmS_. Interestingly, transcription of the _glmY_ gene is regulated by RpoN and RpoD sigma factors using the same transcription start site in an analogous manner to the transcriptional regulation of _rpoE_P2 and _rpoE_P3 promoters [11; 28]. RpoN-regulated promoters of _glmY_ and _rpoE_ genes use QseF as an activator and thus this mode of regulation may be important to sense common signals and ensure cellular homeostasis in response to envelope stress. Coupled Regulation of LPS and Phospholipids-Regulation of Amounts of Kdo\({}_{2}\)-Lipid A Synthesis  
Regulation of LpxC occurs by regulated proteolysis mediated by FtsH [29; 30]. This proteolysis by FtsH requires the LPS assembly factor LapB [9; 31]. Both FtsH and LapB are essential for bacterial growth and their depletion causes increased synthesis of LPS at the expense of phospholipids. This is due to stabilization of LpxC in either an _ftsH_ or a _lapB_ mutant, which causes diversion of a common precursor \(R\)-3-hydroxynristoyl-ACP towards the LPS synthesis, since LpxC and FabZ compete for the same precursor, limiting the availability of phospholipids (Figure 1). Consistent with the notion of coupling of phospholipid and LPS synthesis, suppressors mapping to the _fabZ_ gene, like _sfhc_21 that encodes a hyperactive variant of FabZ, can bypass the essentiality of either _ftsH_ or _lapB_ genes [9]. Additional evidence supporting this model is based on observations that inhibition of LpxC can be compensated by mutations that compromise the FabZ activity [32]. Similarly, overexpression of the _fabZ_ gene is accompanied by an upregulation of the LpxC activity and vice versa [32]. Further supporting regulated LPS and phospholipid biosynthesis, an overexpression of noncoding sRNA _slrA_ can bypass the lethality of the essential _lapB_ gene [9]. The molecular basis of this suppression was attributed to translational repression of the gene encoding the most abundant protein Lpp, also called [PERSON]'s lipoprotein, with an abundance of \(7\times 10^{5}\) molecules per cell. Hence, SlrA is also called MicL [33]. Each Lpp molecule has three acyl chains {phosphatidylglycerol moieties (PG)} and therefore, when the Lpp amount is reduced due to overexpression of _slrA_ sRNA, it causes an increase in the amount of PG that can restore a balance between phospholipids and LPS (Figure 1). SlrA can also act as a negative regulator of RpoE in a feedback manner, as its overproduction reduces the RpoE activity elevated due to LPS defects in _mlapB_ mutants [9]. The gene encoding SlrA sRNA is transcribed from the RpoE-regulated promoter located within the _cutC_ gene. This 80-nt sRNA is synthesized as a 307 nt precursor mRNA that is processed and is located within the 3\({}^{\prime}\) end of the coding region of the _cutC_ gene [9; 33]. Although a role for LapB and FtsH for LpxC is now known, however how the proteolytic activity of FtsH is regulated remains to be elucidated.