PGN_0297 is an essential component of the type IX secretion system (T9SS) in Porphyromonas gingivalis: Tn‐seq analysis for exhaustive identification of T9SS‐related genes

The type IX secretion system (T9SS) was originally discovered in Porphyromonas gingivalis, one of the pathogenic bacteria associated with periodontal disease and is now known to be present in many members of the phylum Bacteroidetes. The T9SS secretes a number of potent virulence factors, including the highly hydrolytic proteases called gingipains, across the outer membrane in P. gingivalis. To understand the entire machinery of T9SS, an exhaustive search for T9SS‐related genes in P. gingivalis using the mariner family transposon (Tn) and Tn‐seq analysis was performed. Seven hundred and two Tn insertion sites in Tn mutants with no colony pigmentation that is associated with Lys‐gingipain (Kgp) defectiveness were determined, and it was found that the Tn was inserted in the kgp gene and 54 T9SS‐related candidate genes. Thirty‐three out of the 54 genes were already known as T9SS‐related genes. Furthermore, deletion mutant analysis of the remaining 21 genes revealed that they were not related to the T9SS. The 33 T9SS‐related genes include a gene for PGN_0297, which was found to be associated with the T9SS components PorK and PorN. A PGN_0297 gene deletion mutant was constructed, and it was found that the mutant showed no colony pigmentation, hemagglutination or gingipain activities, indicating that PGN_0297 was an essential component of the T9SS. The 33 genes did not include the six genes (gppX, omp17, porY, rfa, sigP and wzx) that were also reported as T9SS‐related genes. gppX deletion and insertion mutants were constructed, and it was found that they did not show deficiency in the T9SS.