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Background  Intracellular zinc concentration needs to be maintained within strict limits due to its toxicity at high levels, and this is achieved by a finely regulated balance between uptake and efflux. Many bacteria use the Zinc Uptake Regulator Zur to orchestrate zinc homeostasis, but little is known regarding the transport of this metal across the bacterial outer membrane.    Results  In this work we determined the  Caulobacter crescentus  Zur regulon by global transcriptional and  in silico  analyses. Among the genes directly repressed by Zur in response to zinc availability are those encoding a putative high affinity ABC uptake system ( znuGHI ), three TonB-dependent receptors ( znuK ,  znuL  and  znuM ) and one new putative transporter of a family not yet characterized ( zrpW ). Zur is also directly involved in the activation of a RND and a P-type ATPase efflux systems, as revealed by β-galactosidase and site-directed mutagenesis assays. Several genes belonging to the Fur regulon were also downregulated in the  zur  mutant, suggesting a putative cross-talk between Zur and Fur regulatory networks. Interestingly, a phenotypic analysis of the  znuK  and  znuL  mutants has shown that these genes are essential for growth under zinc starvation, suggesting that  C. crescentus  uses these TonB-dependent outer membrane transporters as key zinc scavenging systems.    Conclusions  The characterization of the  C. crescentus  Zur regulon showed that this regulator coordinates not only uptake, but also the extrusion of zinc. The uptake of zinc by  C. crescentus  in conditions of scarcity of this metal is highly dependent on TonB-dependent receptors, and the extrusion is mediated by an RND and P-type ATPase transport systems. The absence of Zur causes a disturbance in the dynamic equilibrium of zinc intracellular concentration, which in turn can interfere with other regulatory networks as seen for the Fur regulon.    Electronic supplementary material  The online version of this article (doi:10.1186/1471-2164-15-734) contains supplementary material, which is available to authorized users.

Analysis of the Caulobacter crescentus Zur regulon reveals novel insights in zinc acquisition by TonB-dependent outer membrane proteins