Identification of PGN_1123 as the Gene Encoding Lipid A Deacylase, an Enzyme Required for Toll-Like Receptor 4 Evasion, in Porphyromonas gingivalis

Removal of one acyl chain from bacterial lipid A by deacylase activity is a mechanism used by many pathogenic bacteria to evade the host's Toll-like receptor 4 (TLR4)-mediated innate immune response. In Porphyromonas gingivalis, a periodontal pathogen, lipid A deacylase activity converts a majority of the initially synthesized penta-acylated lipid A, a TLR4 agonist, to tetra-acylated structures, which effectively evade TLR4 sensing by being either inert or antagonistic at TLR4. In this paper, we report successful identification of the gene that encodes the P. gingivalis lipid A deacylase enzyme. This gene, PGN_1123 in P. gingivalis 33277, is highly conserved within P. gingivalis, and putative orthologs are phylogenetically restricted to the Bacteroidetes phylum. Lipid A of APGN_1123 mutants is penta-acylated and devoid of tetra-acylated structures, and the mutant strain provokes a strong TLR4-mediated proinflammatory response, in contrast to the negligible response elicited by wild-type P. gingivalis. Heterologous expression of PGN_1123 in Bacteroides thetaiotaomicron promoted lipid A deacylation, confirming that PGN_1123 encodes the lipid A deacylase enzyme. IMPORTANCE Periodontitis, commonly referred to as gum disease, is a chronic inflammatory condition that affects a large proportion of the population. Porphyromonas gingivalis is a bacterium closely associated with periodontitis, although how and if it is a cause for the disease are not known. It has a formidable capacity to dampen the host's innate immune response, enabling its persistence in diseased sites and triggering microbial dysbiosis in animal models of infection. P. gingivalis is particularly adept at evading the hosts TLR4-mediated innate immune response by modifying the structure of lipid A, the TLR4 ligand. In this paper, we report identification of the gene encoding lipid A deacylase, a key enzyme that modifies lipid A to TLR4-evasive structures.