# Regulated Assembly of LPS, Its Structural Alterations and Cellular Response to LPS Defects
## 9 Signal Transduction in Response to LPS Defects  
Integrity of the OM and homeostasis of various overall cell envelope components is critical for growth and the viability of bacteria. This requires correct assembly of OMPs, a balance between synthesis of peptidoglycan, phospholipids and LPS. This in bacteria requires function of various regulon members of RpoE and Cpx TCS. Severe defects in LPS biogenesis in response to underacylation, defects in LPS when the LPS assembly factor LapB is missing or depletion of Lpt translocation system, truncation in the inner core or when LPS synthesis is compromised particularly when bacteria synthesize minimal LPS derivatives like Kdo\({}_{2}\)-lipid A, Kdo\({}_{2}\) lipid IV\({}_{\text{A}}\) or only lipid IV\({}_{\text{A}}\) derivatives, cause major cell envelope perturbations leading to a signal response that causes activation of the RpoE sigma factor, induction of the Rcs phosphorelay and activation of the Cpx TCS [7; 9; 10; 11]. The extent of induction of these stress responses varies depending upon the severity of defects. Early work addressing activation of RpoE, when the OMP assembly is compromised, revealed that strains lacking GmbH (HtrM), which synthesize Kdo\({}_{2}\)-lipid A Re LPS, overexpress exopolysaccharideandide and have a constitutive elevation of RpoE response with a concomitant decrease in OMP content [17]. The exopolysaccharideandide synthesis is known to be positively regulated by the Rcs TCS by increasing transcription of _wca_ genes [80]. Subsequently, during deciphering of the RpoE regulon, it was revealed that some of the genes involved in either LPS biosynthesis, LPS modifications, LPS transport and factors involved in OMP folding and insertion in the OM are part of this regulon [14]. In further studies, detailed mutational analysis of various genes, whose products are either involved in LPS biosynthesis or LPS modifications, coupled with mass spectrometric analysis of LPS of all such mutant derivatives and an impact on envelope stress response was undertaken in series of studies [7; 9; 11]. These studies reveal a full circuit wherein defects in LPS cause a major induction of RpoE and RpoE induction without any mutations in LPS biosynthetic/assembly pathways lead to several alterations leading to the accumulation of specific LPS glycoforms and various modifications (Figure 3). Some of the major conclusions and major pathways of stress response are discussed here. These studies showed that underacylation of LPS, when the core biosynthetic pathway is intact, does not cause any major induction of either RpoE or Cpx systems [7]. Only when all three acyltransferases are deleted, as in a strain \(\Delta\)(_lpxL lpxM lpxP_), a modest induction of RpoE-dependent pathway is induced. In contrast, \(\Delta\)_wca_C strains synthesizing Kdo\({}_{2}\)-lipid A LPS exhibit a nearly 3-fold induction of RpoE pathway without impactingthe Cpx system. However, tetraacylated derivatives synthesizing Kdo\({}_{2}\)-lipid IV\({}_{\rm A}\) like \(\Delta\)(_vaaC lpxL_) or \(\Delta\)(_vaaC lpxL lpxM lpxP_) exhibit a 3-4-fold induction of both RpoE and Cpx pathways even under permissive growth conditions. Consistent with these are the findings that \(\Delta\)_vaaA_ mutants synthesizing lipid IV\({}_{\rm A}\) precursor species exhibit maximal induction of RpoE and Cpx pathways. Given much higher induction of RpoE in \(\Delta\)_vaaC_ mutants and lipid IV\({}_{\rm A}\) synthesizing strains, as compared to its modest induction in \(\Delta\)(_lpxL lpxM lpxP_) mutants, suggests an acute requirement for the incorporation of Kdo and initial steps in LPS core biosynthesis for cell envelope integrity. Interestingly, a \(\Delta\)(_lpxL lpxM lpxP_) strain was found to synthesize LPS primarily of glycoporm with a third Kdo residue and a truncation of outer core disaccharide, when grown in lipid A modifying conditions [7].