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

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

Methods: The role of Crp protein in was determined using the Δ mutant strains to characterize their phenotype and to assess the impact of inactivation on gene expression using RNA-seq and RT-qPCR. Additionally, the Crp protein was purified and characterized.

Results: Key findings in the Δ mutant strain include up-regulated and genes and down-regulated , and genes. While inactivation does not affect growth in liquid culture media, it impairs biofilm formation and enhances adhesion to and invasion of gingival keratinocytes. Crp binds directly to its own and promoters without interacting with cyclic nucleotides or di-nucleotides. Its three-dimensional structure, resembling Crp in complex with cAMP and DNA, suggests that Crp functions as a global regulator independently of cAMP binding. The highest 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 protein amount.

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