Iron homeostasis positively influences copper tolerance, promoted by TbcT and OxcT in Caulobacter crescentus

The copper tolerance of the free-living bacteria Caulobacter crescentus depends on its dimorphic cell cycle. The sessile stalked cell detoxifies and effluxes copper through the multicopper oxidase PcoA and the Cu transporter PcoB respectively, while the swarmer cell senses and swims away from Cu sources. This is further confirmed by transcriptional landscape of both morphotypes upon copper excess. Among the few genes upregulated in both stalked and swarmer cells under copper excess, the CCNA_00027-00028 operon encodes a TonB-dependent receptor (TbcT) and a 2-oxoglutarate/Fe²⁺-dependent oxygenase (OxcT), respectively. The deletion of these two genes specifically increases the sensitivity towards Cu in C. crescentus. Interestingly, using a bioinformatics approach, we observed that this system is conserved in at least 67 % of TbcT-containing bacteria, and 88 % of OxcT-containing bacteria. The TbcT-OxcT system is not involved in the transport of copper nor in the detoxification of copper-induced oxidative stress. Previous studies in C. crescentus and for homologous proteins showed that TbcT seems to be involved in iron import via siderophores, even though C. crescentus does not appear to synthetize siderophores. The overexpression of the tbcT gene appears to enhance the import of iron. OxcT activity is important for copper tolerance, although its specific activity has not yet been deciphered. Based on our results, we demonstrated that proper iron import is crucial for complete Cu tolerance, with the intracellular iron preventing copper accumulation. Taken together, our results argue for a tight coupling between iron and copper homeostasis in the context of copper tolerance.