**7.1. Regulation of the biosynthesis of lipid A**

Lipid A and the core oligosaccharide are formed in a continuous process, which is separate from the synthesis of the O antigen. In the majority of *Enterobacteriaceae*, the genes involved in this synthesis are found in a single copy and share several characteristics with pathogenicity islands (PAI) [10,24].

The genes involved in the first steps of the biosynthesis of lipid A in *E. coli, S. enterica*  serovar Typhimurium, *Yersiia enterocolitica*, *Haemophilus influenzae* and *Rickettsia rickettsii* are grouped into the *lpx*D-*fab*Z*-lpx*A-*lpx*B cluster[8]. The genes *lpx*A and *lpx*D code for Nacyltransferases, which add fatty acids to the glucosamine disaccharide. Both enzymes contain a conserved repeated structure, which is the hexapeptide [(I,V,L)GXXXX]n. The gene *lpx*B is a co-transcript with *lpx*A and codes for the disaccharide synthase of lipid A, which catalyses the formation of the disaccharide of lipid A from UDP-2.3-diacylglucosamine and 2,3-diacylglucosamine-1-phosphate. The gene *fab*Z codes the enzyme that catalyses the dehydration of (3R)-hydroxyacyl-ACP to trans-2-acyl-ACP, which is used as a donor of fatty acid in the biosynthesis of phospholipids [10].

The proteins involved in the biosynthetic pathways of UDP-GlcNAc, UDP-Glc and UDP-Gal are coded in constitutive genes.

L-glycero-D-manno-heptose is added to its derivative ADP, which is synthesised from sedoheptulose 7-phosphate in four steps. The genes *gmhA* and *gmhD* code for enzymes in the first and last steps. The G+C content of the *gmhA*, *gmhD* and *waaE* genes are 51%, 51% and 52.7%, respectively.

Kdo is transferred from CMP-Kdo and synthesised from arabinose-5P and PEP by a threestage pathway[9]. Two of the genes in this process, namely, *kdsA* and *kdsB*, are wellcharacterised and have G+C contents of 51.6% and 52.7%, respectively.

The gene *waaA*, which is located in the *waa* cluster, codes for a bifunctional Kdo transferase that adds residues from 2 Kdo. These genes have a G+C content of between 51% and 54%. Several of the modifications of lipid A are regulated by the concentration of Mg2+ through the regulon *phoP-phoQ* [25].

PhoP-PhoQ is a two-component system that regulates virulence through adaptation to limited magnesium environments and regulates numerous cellular activities in gramnegative bacteria. This regulon consists of an external membrane sensor, PhoQ, and a cytoplasmic regulator, PhoP, and is activated by the acidic pH and by certain antimicrobial peptides (APs). PhoP-PhoQ is repressed by millimolar concentrations of magnesium and calcium. PhoQ senses the concentration of magnesium and of APs throughout a periplasmic domain, which undergoes a conformational change when it is joined to these compounds and results in autophosphorylation. The activation of the PhoP-PhoQ system may allow for the activation or repression of 40 genes [26].

The regulon PrmAB of *Salmonella* is also a two-component system coded in the operon *pmrCAB*, which has protein products that include a phosphoethanolamine phosphotransferase (PmrC), a response regulator (PmrA) and a kinase sensor (PmrB). It has been confirmed that PmrAB regulates more than 20 genes in *Salmonella*. However, several studies suggest that more than 100 genes show activity[27]. One of the primary roles of the activation of PmrAB is the modification of LPS, such as the addition of Ara4N to lipid A, which, as explained below, impacts the susceptibility to some antimicrobial agents and the addition of phosphoethanolamine (pEtN) in the core of the LPS [26].
