# Regulated Assembly of LPS, Its Structural Alterations and Cellular Response to LPS Defects
## 3 Essentiality of LPS and the Minimal LPS Structure  
Generally, LPS is essential for the viability of vast majority of Gram-negative bacteria with few exceptions. Few limited exceptions include viable LPS-lacking mutants of _Acinetobacter baumannii_ that have mutations in either _lpxA_ or _lpxC_ or _lpxD_ genes [15]. However, in such mutant strains, the absence of LPS is compensated by increased expression of the Lol lipoprotein transport system to increase phospholipid export, enhanced expression of poly-\(\beta\)-1,6-\(N\)-acetylglucosamine and elevated expression Mla-retrograde phospholipid migration from the OM to the IM to maintain a balance in the essential constituents of the OM [15]. Another well-studied example of dispensability of LPS include construction of viable LPS deficient mutants of _Neisseria meningitides_[16]. It is well established that in bacteria, like _E. coli_ and _Salmonella_, the minimal structure of LPS that can sustain the bacterial viability up to 42 \({}^{\circ}\)C is composed of hexaacylated lipid A-Kdo\({}_{2}\) (Re LPS \"deep-rough mutants\"). Thus, \(\Delta\)_vwaaC_ (lacking hepatocellular transferase I) or \(\Delta\)_gmlD_ (absence of ADP-\(\iota\)-_glycero-\(\upsilon\)-manno_-heptose-6-epimerase) mutants are viable, although they exhibit temperature sensitive growth (unable to grow at temperatures above 43 \({}^{\circ}\)C), permeability defects, hypersensitivity to detergents, hyperactivated RpoE-regulated stress response, inability to colonize the host, sensitivity to antimicrobial peptides, defects in flagellar biosynthesis, compromised growth at low pH, and a constitutive induction of Rcs-dependent exopolysaccharide [7; 17; 18; 19; 20; 21]. Consistent with the requirement of inner core heptose attachment to Kdo\({}_{2}\)-lipid A for cell envelope integrity, a \(\Delta\)(_vwaC surA_) mutational  
Figure 1: Networks of regulatory pathways that control the lipopolysaccharide-acrylamide (LPS) assembly and its non-stoichiometric modifications. The RpoE sigma factor responds to severe defects in LPS biosynthesis and is also required for transcription of genes involved in LPS biosynthesis/translocation and modifications via sRNAs like _rybB_ and _micA_. RpoE also transcribes the _slrA_ (_micL_) sRNA, which represses the Lpp synthesis and acts in a feedback manner to repress RpoE. Other regulatory controls involve two-component systems like BasS/R, PhoP/Q, PhoB/R and Rcs system, which are required for transcription of genes whose products are involved in lipid A and inner core modifications. The RpoH heat shock sigma factor transcribes _lapB_ and _ftsH_ genes, whose products control balanced biosynthesis of LPS and phospholipid by regulating LpxC levels. The unique transcriptional factor RfaH is required for overcoming antitermination, enhance transcriptional elongation and couple transcription/translation of _vwaQ_ and _rfb_ LPS biosynthetic operons. The sRNA RirA binds to RfaH and abrogates its activity to maintain a balanced biosynthesis of LPS. combination confers synthetic lethality [7]. SurA is a major periplasmic folding factor required for the folding of outer membrane proteins (OMPs), including LptD [17; 22]. Furthermore, the lack of other conserved core biosynthetic genes like _waaG_, _waaF_ or genes involved in the pathway of synthesis of molecules of _t-glycero-_\(\alpha\)-_p-mano_-heptose (heptose) such as _gmhA_ and _gmhE_, also results in impairment of growth at high temperatures [23]. A deep-rough phenotype accompanied by hypersensitivity to detergents and antibiotics is also associated with the lack of WaaP kinase, which mediates phosphorylation of HepI and completion of core biosynthesis [24] (Figure 1). Underacylation of lipid A due to a lack of the LpxL Lauroyl transferase is known to confer temperature sensitivity above 33 \({}^{\circ}\)C and strains lacking all three acyltransferases \(\Delta\)(_lpxL lpxP lpxM_) cannot grow at even 30 \({}^{\circ}\)C on rich medium [5].