The modifications include the PhoP/PhoQ-regulated expression of pagP , which is involved in the acylation at position 2 of the proximal unit of lipid-A ( Bishop , 2005 ) , and the PmrA/PmrB-controlled expression of the factors required for the addition of 4-amino-4-deoxy-L-arabinose ( L-Ara4N ) and phosphoethanolamine ( pEtN ) to phosphate groups of the lipid-A ( Groisman et al. , 1997 ; Gunn et al. , 2000 ; Lee et al. , 2004 ) .