GacA regulates symbiosis and mediates lifestyle transitions in Pseudomonas

Through horizontal gene transfer, closely related bacterial strains have assimilated distinct sets of genes, resulting in significantly varied lifestyles. However, it is not clear how strains are able to properly regulate horizontally transferred virulence genes. We hypothesized that strains may use components of the core genome to regulate diverse horizontally acquired genes. To investigate how closely related bacteria assimilate and activate horizontally acquired DNA, we used a model of Pseudomonas fluorescens subspecies brassicacearum strains, N2E2 and N2C3, which exhibit contrasting lifestyles on the model plant Arabidopsis. P. brassicacearum strain N2E2 is a plant commensal and contains the genes that encode biosynthetic enzymes for the anti-fungal compound 2,4-Diacetylphloroglucinol (DAPG). In contrast, N2C3 lacks DAPG biosynthesis and has gained a pathogenic island encoding syringomycin (SYR)-and syringopeptin (SYP)-like toxins from the plant pathogen P. syringae. This causes a transition in lifestyle from plant-protective N2E2 to plant pathogenic N2C3. We found that N2E2 and N2C3 share a highly conserved two-component system GacA/S, a known regulator of DAPG and SYR/SYP. Using knockout mutations, we found that a{Delta} gacA mutation resulted in loss of expression of SYR/SYP virulence genes and returned pathogenic N2C3 to a plant commensal lifestyle. Our study further explored the conservation of regulatory control across strains by demonstrating that GacA genes from both distant and closely related Pseudomonas strains could functionally complement one another across the genus. IMPORTANCEEmerging pathogens represent a significant threat to humans, agriculture, and natural ecosystems. Bacterial horizontal gene transfer aids in the acquisition of novel genes that facilitate bacterial adaptation to new environments. Our work shows a novel role for GacA in orchestrating the regulatory changes necessary for virulence and lifestyle transitions facilitated by HGT. These findings suggest that the GacA/S system plays a key role in mediating transitions across diverse Pseudomonas symbiotic lifestyles. It provides insights into the mechanisms that drive the emergence of pathogenic strains and highlights potential targets for managing bacterial threats to plant health.