GacA regulates symbiosis and mediates lifestyle transitions in Pseudomonas

Through horizontal gene transfer, closely related bacterial strains assimilate distinct sets of genes, resulting in significantly varied lifestyles. However, it remains unclear how strains 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 consisting of strains in the brassicacearum/corrugata/mediterranea (BCM) subclade of Pseudomonas fluorescens, including Pseudomonas species N2E2 and N2C3, which exhibit contrasting lifestyles on the model plant Arabidopsis. Pseudomonas sp. N2E2 is a plant commensal and contains genes encoding biosynthetic enzymes for the antifungal compound 2,4-diacetylphloroglucinol (DAPG). In contrast, Pseudomonas sp. N2C3 lacks DAPG biosynthesis and has gained a pathogenic island encoding syringomycin (SYR)- and syringopeptin (SYP)-like toxins from the plant pathogen Pseudomonas 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 ΔgacA 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.