# Regulated Assembly of LPS, Its Structural Alterations and Cellular Response to LPS Defects
## 8 Regulated Structural Alterations in LPS
### Glycoform Switches  
Up to now, at least seven structurally different glycoforms of _E. coli_ K-12 have been characterized, whose relative abundance varies on growth conditions such as exposure to phosphate-limiting growth  
Figure 2: Schematic depiction of unmodified hexaacylated glycoform I and glycoform derivatives with various non-stoichiometric substitutions with a third Kdo-Rho disaccharide. Various genes whose products mediate different steps in LPS biosynthesis and incorporation of different modifications are indicated. Glycoform I constitutes the major LPS species under non-stress conditions (**A**). Upon the induction of RpoE sigma factor and conditions inducing Bass/R and PhoB/R TCSs, the lipid A part is modified by P-EtN and L-Ara4N at 1 and 4\({}^{\prime}\) position by EptA and ArnT, respectively (**B**). The induction of RpoE causes increased synthesis of EptB overcoming MgrR silencing and incorporation of P-EtN on the second Kdo (panels B1 and B2). The induction of RpoE favors pathway of the third Kdo incorporation by increased synthesis of WaaZ and repression of WaaR by the RybB sRNA, leading to synthesis of glycoforms with a third Kdo with attachment of Rha on the third Kdo with a concomitant truncation of the terminal GlcIII-HepIV disaccharide shown in violet background. The PhoB/R induction leads to the incorporation of GlcUA at the expense of HepII phosphorylation (blue background). Lipid A can also be modified by PagP generating heptaacylated lipid A and by the removal of acyl chains by LpxR in the OM after translocation (light brown background). Various sRNA-mediated controls are shown in blue color. conditions, the induction of RpoE-regulated envelope stress response and activation of specific TCSs. These glycoforms differ due to the non-stoichiometric modifications such as P-EtN transfer to the second Kdo, incorporation of a third Kdo, addition of sugars like Rha, GlcN, uronic acids, alterations in the numbers of phosphate residues in the LPS core and truncation of outer core terminal disaccharide. In initial studies using optimal growth conditions, the wild-type _E. coli_ K-12 was found to contain majority of LPS corresponding to glycoform I structure and only minor amounts of three additional glycoforms II, III and IV could be observed [76]. Glycoform I contains two Kdo residues in the inner core, and four heptoses and four hexoses attached in specific order in the inner core and the outer core (Figure 2). However, using strains or growth conditions that exhibit the induction of RpoE sigma factor or employing phosphate-limiting growth conditions supplemented with Fe\({}^{3+}\) and Zn\({}^{2+}\) (induction of PhoB/R and BasS/R TCSs) or treatment with AMV (induction of RpoE and non-specific induction of TCSs) revealed major shifts in the LPS composition and prevalence of different glycoforms not only in _E. coli_ K-12 but also in _E. coli_ strains with different core type and in _Salmonella_[55; 62]. Using growth medium that induces BasS/R and PhoB/R TCSs, wild-type _E. coli_ strains synthesize more glycoform IV/V derivatives as compared to glycoform I. However, when the RpoE induction is maximal, a near-exclusive synthesis of glycoform V and its derivatives are observed (Figure 2). This molecular switch to the synthesis of glycoform V derivatives requires ppGpp alarmone, induction of the RpoE-transcribed genes _eptB_, sRNAs _micA_ and _rybB_, and the transcriptional upregulation of _waaZ_ with a concomitant repression of WaaR synthesis [62] (Figure 2). _waaZ_ and _waaS_ genes encode the Kdo transferase required for the incorporation of the third Kdo and rhamnyl transferase, respectively, while as the _eptB_ gene encodes P-EtN transferase specific to the second Kdo [62; 77] (Figure 2).