cAMP-independent Crp homolog adds to the multi-layer regulatory network in Porphyromonas gingivalis

Porphyromonas gingivalis encodes three CRP/FNR superfamily proteins: HcpR, PgRsp, and Crp^Pg, with Crp^Pg similar to cAMP-sensing proteins but not classified into known families. This study investigates the role of Crp^Pg in regulating the expression of factors essential for P. gingivalis virulence in A7436 and ATCC 33277 strains.

The role of Crp^Pg protein in P. gingivalis was determined using the Δcrp mutant strains to characterize their phenotype and to assess the impact of crp inactivation on gene expression using RNA-seq and RT-qPCR. Additionally, the Crp^Pg protein was purified and characterized.

Key findings in the Δcrp mutant strain include up-regulated mfa1-5 and rgpA genes and down-regulated trxA, soxR, and ustA genes. While crp inactivation does not affect growth in liquid culture media, it impairs biofilm formation and enhances adhesion to and invasion of gingival keratinocytes. Crp^Pg binds directly to its own and mfa promoters without interacting with cyclic nucleotides or di-nucleotides. Its three-dimensional structure, resembling E. coli Crp in complex with cAMP and DNA, suggests that Crp^Pg functions as a global regulator independently of cAMP binding. The highest crp expression in the early exponential growth phase declines as cell density and metabolic conditions change over time, suggesting a regulatory function depending on the Crp^Pg protein amount.

By controlling the shift from planktonic to biofilm lifestyle, Crp^Pg may play a role in pathogenicity. Regulating the expression of virulence factors required for host cell invasion and intracellular replication, Crp^Pg may help P. gingivalis evade immune responses.