Abstract Polymicrobial infections are inherently more complex than monospecies infections, and this complexity has been a significant barrier to both a fuller appreciation of pathogenic mechanisms, and to the development of effective measures to control or prevent the disease. Periodontal diseases typify polymicrobial diseases, and are among the most common infections of humans. Although certain organisms, such as Porphyromonas gingivalis, are considered key pathogens, it is the polymicrobial community as a whole that initiates and drives the disease. Interspecies interactions within communities shape overall pathogenic potential, and work supported by this project has uncovered a dynamic and multilevel interaction between P. gingivalis and its community partners. The pathogenicity of P. gingivalis can be constrained by the streptococcal diffusible metabolite, para-amino benzoic acid (pABA), which is as a precursor of de novo folate synthesis in the physiologically essential one-carbon metabolism (OCM) pathway. However, as physically integrated communities develop, interspecies co-adhesion results in suppression of streptococcal pABA production, and additionally, partner organisms such as Streptococcus gordonii and Fusobacterium nucleatum, enhance overall community pathogenicity. Sensing of both pABA and interspecies physical contact by P. gingivalis is transduced through a protein tyrosine phosphorylation signaling circuit. The tyrosine kinase Ptk1 controls flux through OCM as well as the production of virulence factors, including the gingipain proteases which can modulate interactions with neutrophils. Indeed, Ptk1 is required for pathogenicity in both murine abscess and alveolar bone loss models of disease. In this proposal we will define the role of Ptk1 as a master regulator which can integrate OCM with pathogenicity. In the first Aim we will examine how Ptk1-mediated phosphorylation of gingipains is necessary for their processing and secretion. We will determine where non-phosphorylated gingipains remain localized in P. gingivalis and investigate which components of the type IX secretion system machinery malfunction in the absence of phosphorylation. Additionally, we will follow post-translational maturation of non-phosphorylated gingipains to uncover the role of phosphorylation in catalytic removal of the N-terminal peptide, the haemagglutinin domains, or the C- terminal domain as is required for the assembly of fully activated proteases. In the second Aim, we shall functionally dissect the contribution of phosphorylation to the activity of the pABA synthesis enzyme PabC, and decipher a negative feedback loop whereby pABA can inhibit Ptk1. Aim 3 will address the impact of Ptk1 on the activation state of neutrophils and their potential for tissue destructive responses to P. gingivalis. Completion of this project will provide novel insights into the context-dependent pathophysiology of P. gingivalis, which could ultimately be translated into...