Investigation of Zur-regulated metal transport systems reveals an unexpected role of pyochelin in zinc homeostasis

Abstract

AbstractLimiting the availability of transition metals at infection sites serves as a critical defense mechanism employed by the innate immune system to combat microbial infections.Pseudomonas aeruginosaexhibits a remarkable ability to thrive in zinc-deficient environments, which is facilitated by intricate cellular responses governed by numerous genes regulated by the zinc-responsive transcription factor Zur. Many of these genes have unknown functions, including those within the predictedPA2911-PA2914andPA4063-PA4066operons. A bioinformatic analysis revealed thatPA2911-PA2914comprises a TonB-dependent outer membrane receptor and an inner membrane ABC-permease responsible for importing metal-chelating molecules, whereasPA4063-PA4066contains genes encoding a MacB transporter, likely involved in the export of large molecules. Molecular genetics and biochemical experiments, feeding assays, and intracellular metal content measurements demonstrated thatPA2911-PA2914andPA4063-PA4066are engaged in the import and export of the pyochelin-cobalt complex, respectively. Notably, cobalt can reduce zinc demand and promote the growth ofP. aeruginosastrains unable to import zinc, highlighting pyochelin-mediated cobalt import as a novel bacterial strategy to counteract zinc deficiency. These results unveil an unexpected role for pyochelin in zinc homeostasis and challenge the traditional view of this metallophore exclusively as an iron transporter.Author SummaryMetals such as iron and zinc are essential nutrients for microbial cells. Consequently, the vertebrate innate immune system employs mechanisms to sequester these elements, counteracting the proliferation of pathogenic bacteria.Pseudomonas aeruginosa, an opportunistic pathogen responsible for threatening infections in immunocompromised individuals, demonstrates a particular ability to resist the zinc sequestration mechanisms implemented by the host. The ability of the bacterium to proliferate in zinc-poor environments is linked to the activation of a large number of genes induced by zinc deficiency, many of which still have unknown functions. Among these genes are those grouped into two clusters, namelyPA2911-2914andPA4063-PA4066. The data reported in our study demonstrate that these two gene clusters encode systems for importing and exporting cobalt-bound pyochelin, respectively. This is a useful strategy to counteract conditions of severe zinc deficiency, as cobalt can replace zinc in many proteins. This discovery prompts us to propose a reassessment of the paradigm stating that pyochelin is exclusively involved in iron acquisition. Indeed, our study reveals the involvement of pyochelin in cellular responses to zinc deficiency, suggesting that this molecule has a broader role in connecting the homeostasis of different metals.