A bacterial chaperone-usher pathway links interkingdom association to antimicrobial protection of a fungal partner

Antimicrobial compounds (AMCs) influence species-level composition of microbial communities, governing beneficial and pathogenic interactions with plant and mammalian hosts. Phenazine-1-carboxylic acid (PCA) is a bacterially produced, redox-active AMC abundant in the rhizosphere. A recent study discovered the PCA-susceptible fungus Aspergillus calidoustus can grow in PCA-abundant environments by associating with phenazine-tolerant bacteria, including the novel species Paraburkholderia edwinii . P. edwinii's protection response is characterized by adherence to its fungal partner, bacterial aggregation, and sequestration and reduction of PCA, rendering PCA less toxic. Deletion of the bacterial transcriptional repressor HrcA generates a constitutively active protection response and enhanced protection of A. calidoustus . Here, we use the ΔhrcA strain to determine whether fungal protection uses bacterial self-protection machinery or dedicated interkingdom mechanisms. We identified a divergent archaic chaperone-usher pathway (CUP) operon encoding a predicted periplasmic chaperone, outer membrane usher, putative major pilin-like subunit, and putative adhesin-like tip protein. While many archaic CUP operons are annotated in diverse bacteria, their functions remain largely unexplored. We show these fungal attachment complex ( fac ) genes promote bacterial-fungal attachment, are required for P. edwinii to sequester and detoxify PCA, and contribute to full fungal protection from PCA stress, despite not being required for P. edwinii's own tolerance of PCA. We further identified the transcription regulator, FacR, that influences expression of the fac system. Understanding the functions of divergent archaic CUP systems may illuminate protective interkingdom partnerships and inform how pathogenic and beneficial microbes gain entry to AMC-replete environments, shaping microbial communities in agricultural and clinical contexts.