Control of Biofilm Formation by an Agrobacterium tumefaciens Pterin-Binding Periplasmic Protein Conserved Among Pathogenic Bacteria
Biofilm formation and surface attachment in multiple Alphaproteobacteria is driven by unipolar polysaccharide (UPP) adhesins. The pathogen Agrobacterium tumefaciens produces a UPP adhesin, which is regulated by the intracellular second messenger cyclic diguanylate monophosphate (cdGMP). Prior studies revealed that DcpA, a diguanylate cyclase-phosphodiesterase (DGC-PDE), is crucial in control of UPP production and surface attachment. DcpA is regulated by PruR, a protein with distant similarity to enzymatic domains known to coordinate the molybdopterin cofactor (MoCo). Pterins are bicyclic nitrogen-rich compounds, several of which are formed via a non-essential branch of the folate biosynthesis pathway, distinct from MoCo. The pterin-binding protein PruR controls DcpA activity, fostering cdGMP breakdown and dampening its synthesis. Pterins are excreted and we report here that PruR associates with these metabolites in the periplasm, promoting interaction with the DcpA periplasmic domain. The pteridine reductase PruA, which reduces specific dihydro-pterin molecules to their tetrahydro forms, imparts control over DcpA activity through PruR. Tetrahydromonapterin preferentially associates with PruR relative to other related pterins, and the PruR-DcpA interaction is decreased in a pruA mutant. PruR and DcpA are encoded in an operon that is conserved amongst multiple Proteobacteria including mammalian pathogens. Crystal structures reveal that PruR and several orthologs adopt a conserved fold, with a pterin-specific binding cleft that coordinates the bicyclic pterin ring. These findings define a new pterin-responsive regulatory mechanism that controls biofilm formation and related cdGMP-dependent phenotypes in A. tumefaciens and is found in multiple additional bacterial pathogens. SIGNIFICANCEBiofilms are bacterial communities attached to surfaces, physiologically distinct from free-living cells, and a common cause of persistent infections. Here we define the mechanism of a novel biofilm regulatory system based on excreted metabolites called pterins, that is conserved within a wide range of Gram-negative bacteria, including multiple pathogens of animals and plants. The molecular mechanism of pterin-dependent regulation is reported including structural determination of several members of a new family of pterin-binding proteins. Pterins are produced across all domains of life and mechanistic insights into this regulatory circuit could lead to new advances in antibiofilm treatments.