Identification and characterization of Porphyromonas gingivalis TonB

Porphyromonas gingivalis is one of the major bacterial pathogens responsible for the initiation and progression of periodontal disease. The bacterium codes for multiple TonB-dependent receptors required for acquisition of nutrients such as heme and vitamin B12, although the identity of a TonB energy transducer has yet to be identified. Here we identify a potential TonB protein encoded by PG0785, generate a deficient strain, and characterize its biological significance. Bioinformatics analysis reveals that the PG0785 has unique features confined to the Cytophaga-Flavobacterium-Bacteroides (CFB) group of bacteria but shares similarities in the C-terminal domain (CTD) to well characterized proteins from Helicobacter pylori and Escherichia coli. Similarity at a protein level, as well as the genomic locus organization, was identified with the TonB proteins characterized in other Bacteroidota species. Loss of PG0785 led to significant alteration of the proteome: proteins mediating gene regulation, protein translation, protection against reactive radical species (oxygen and nitrogen), and glycosylation were up-regulated and polysaccharide export, efflux protein, and lipoproteins were downregulated. Furthermore, phenotypic studies showed that a mutant lacking PG0785 cannot accumulate heme on its surface and is deficient in gingipain protease activity. In addition, reduction of the capsular layer was detected in the TonB-deficient strain. Thus, while the mutant interacted and invaded eukaryotic cells at much higher levels than the wild type, it had significantly reduced ability to survive with host cells.

The TonB system is indispensable for energy transduction in Gram-negative bacteria and is mainly associated with nutrient uptake. Although well investigated in Proteobacteria, the knowledge regarding the system in the CFB group of bacteria is lagging. In contrast to Proteobacteria, the intestinal Bacteroidota have multiple TonBs and their novel involvement in sugar utilization has been demonstrated. This study shows that the oral Bacteroidota, Porphyromonas gingivalis , also has TonB but unexpectedly, it is not involved in iron homeostasis or nutrient acquisition but rather in processes involved in maturation of proteases and cell surface remodeling. As maintenance of outer membrane integrity is mainly associated with Tol motor our study shows possible crosstalk between the energy transducers. Also, this study further demonstrates the functional versatility of the TonB system depending on the environmental niche and the metabolic requirements of the bacteria.