# Regulated Assembly of LPS, Its Structural Alterations and Cellular Response to LPS Defects
## 8 Regulated Structural Alterations in LPS
### Regulation of Inner Core Modifications and Switches Between Different Glycoforms  
The inner core of LPS in the majority of cases generally contains an \(\alpha\)-(2-4)-linked Kdo disaccharide, to which l-_glycero_-p-_manno_-heptose residue (Hep) is attached at position 5 of KdoI. In _E. coli_, the inner core contains three Hep residues. Although the composition of the inner core is relatively conserved, Kdo, as well as Hep residues, can be non-stoichiometrically modified. These modifications are regulated by the RpoE sigma factor, PhoP/Q, PhoB/R TCSs, by alarmone ppGpp and recruitment of specific sRNAs, whose expression is regulated by these transcription factors (Figure 1). Like lipid A modifications, they are often important for antibiotic resistance OM permeability and contribute to diversity in the LPS composition [10; 13; 62]. Quite well studied modifications include the non-stoichiometric incorporation of a P-EtN residue on the second Kdo by EptB transferase, incorporation of a third Kdo by WaaZ transferase, the addition of Rha, which can be linked to either the second Kdo or the third Kdo, modification of phosphorylated HepI by P-EtN using the PhoB/R-regulated EptC and incorporation of glucuronic acid (GlcUA) by the PhoB/R-inducible WaaH glycosyltransferase with a concomitant loss of phosphate residue on HepII [55; 62].  
Transcription of the _eptB_ gene encoding P-EtN transferase specific to the second Kdo is positively regulated by the RpoE sigma factor [7; 72]. Hence, this modification of the second Kdo is quite pronounced when the RpoE activity is induced either in the absence of RseA anti-sigma factor or when LPS is defective as in \(\Delta\)_AwaaC_ or \(\Delta\)_AwaF_ mutants [7; 11; 62]. Consistent with the induction of RpoE and regulation of the \(eptB\) transcription, the lipid A part of \(\Delta\)_xwaaC_ or \(\Delta\)_xwaF_ strains were found to lack P-EtN even under \(eptA\)-inducing conditions, but preferentially incorporating P-EtN on the second Kdo [7; 11]. However, this incorporation requires the presence of Ca\({}^{2+}\) in the growth medium. Furthermore, the synthesis of EpitB is negatively regulated by two Hfq-dependent sRNAs MgrR and ArcZ, although in response to different environmental signals [72; 73]. Under normal growth conditions, the PhoP-regulated MgrR sRNA via base-pairing with the \(eptB\) mRNA silences its expression due to translational repression of the \(eptB\) mRNA. Activation of PhoP/Q induces transcription of the \(mgrR\) sRNA. The EptB-dependent P-EtN modification of the second Kdo upon high Ca\({}^{2+}\) concentration can be explained by the repression of transcription of PhoP/Q-regulated \(mgrR\) and promote the synthesis of active EptB under RpoE-inducing conditions. The Hfq-dependent ArcZ sRNA also inhibits translation and expression of \(eptB\) by base-pairing in an ArcA/B-dependent manner in response to oxygen concentration [72]. Quite like the P-EtN modification of lipid A, the incorporation of P-EtN on the second Kdo confers resistance to polymyxin B. Thus, RpoE regulon members like \(eptB\) and MicA sRNA, PhoP-regulated MgrR sRNA contribute to the incorporation of P-EtN on the second Kdo, lipid A and regulation of glycoform switches.