Injectisome assembly primes Pseudomonas aeruginosa for type III secretion

Many Gram-negative pathogens, including Pseudomonas aeruginosa, use a type III secretion system (T3SS) to intoxicate eukaryotic cells. The T3SS is an important virulence factor linked to increased morbidity and mortality in infections, yet its expression slows bacterial growth and activates innate immune receptors. T3SS genes are expressed heterogeneously, with T3SS-ON cells arising from "primed" bacteria that express the T3SS transcriptional activator ExsA and respond immediately to T3SS activating signals. However, the mechanistic basis for priming is not known. ExsA is part of a complex protein-sequestration network, positively regulating its own expression, as well as that of its anti-activator (ExsD), its anti-anti-activator (ExsC), and ExsC's binding partner (ExsE). These four proteins create a bistable regulatory network. We hypothesized that transcription from a cAMP-dependent promoter upstream of ExsA could drive cells into the primed state, and tested this at the single-cell level. Exogenous cAMP increased the proportion of primed, ExsA-expressing cells, with whole-cell cryo-electron tomography demonstrating the assembly of T3SS injectisomes under these conditions. Interstrain variation in endogenous cAMP levels correlated with strain-specific proportions of primed bacteria, while genetic manipulation of cAMP levels altered primed population size. This work demonstrates how endogenous and exogenous cAMP inputs into a bistable regulatory switch generate subpopulations of T3SS-primed cells poised to respond to activating signals.IMPORTANCEType III secretion systems (T3SS) are specialized protein secretion systems that allow bacteria to inject toxins into eukaryotic cells. T3SS are important virulence factors, but their expression carries a fitness cost: they slow bacterial growth and make bacteria vulnerable to detection by the innate immune system. Some pathogens, like Pseudomonas aeruginosa, balance the costs and benefits of T3SS expression by restricting T3SS expression to a subset of cells. T3SS-ON cells arise from "primed" bacteria that express the transcriptional activator ExsA and respond immediately to T3SS activating signals. However, the mechanistic basis for priming is unknown. In this study, we tested whether expression of ExsA from a cAMP-dependent promoter could drive cells into the primed state and found this to be true. Whole-cell cryo-electron tomography demonstrated that primed bacteria assembled T3SS injectisomes. This work demonstrates how cAMP inputs into a bistable regulatory switch generate subpopulations of T3SS-primed cells.