Pseudomonas aeruginosa is an opportunistic pathogen that colonises and infects multiple niches within the body. During infection, P. aeruginosa acquires all essential nutrients, including the d-block metal zinc, from host tissues. It is estimated that ~6% of the bacterial proteome requires zinc for function, highlighting the essentiality of zinc for bacteria viability. Previous studies have shown that the ATP-binding cassette (ABC) permease ZnuBCA is the primary pathway for P. aeruginosa zinc uptake. Deletion of ZnuBCA was associated with the upregulation of putative alternative zinc importers, although their precise contribution to metal homeostasis remains to be defined.
In this study, ZnuBCA and two additional P. aeruginosa zinc import pathways were investigated. The PA4063-66 locus encoded a putative Mac-B type ABC transporter and two periplasmic solute-binding proteins (PA4036, PA4066) that were highly conserved in the Pseudomonad genus. Analyses of recombinant PA4063 and PA4066 revealed that the proteins formed a zinc-binding heterodimeric complex that dissociated at high zinc concentrations. This may suggest a concentration-dependent regulation of zinc transport. Unexpectedly, deletion of all three zinc uptake systems had no apparent impact on bacterial growth phenotypes, planktonic or biofilm, in zinc-limited media. Cellular metal accumulation was only affected by deletion of ZnuBCA, with deletion of the other two transporters having no further impact. However, investigation of the metalloproteome revealed that disruption of the three pathways exerted broader dysregulatory impacts on cellular use of metals.
Collectively, this work reports the unexpected observation that despite the importance of zinc, disruption of P. aeruginosa acquisition does not impair bacterial growth. This is in stark contrast to other pathogens, wherein disruption of zinc homeostasis results in major phenotypic defects. Elucidating the pathways by which P. aeruginosa acquires zinc and mitigates the impact of cellular restriction will reveal the essential cellular processes that contribute to the virulence of this pathogen.