Abstract
Cadmium (Cd) and chromium (Cr) are frequently encountered toxicants, while iron (Fe) plays a crucial role in bacterial survival under conditions of heavy metal stress. However, intracellular Fe ion depletion by heavy metals leads to a state of Fe starvation. Therefore, it is imperative to investigate the mechanism through which bacteria maintain a balance between heavy metal detoxification and Fe homeostasis. This study demonstrates Cd(II) immobilization and Cr(VI) reduction abilities of Stenotrophomonas sp. SY1, while proteomics reveals the upregulation of heme metabolism in response to Cd(II) and Cr(VI) exposure. The expression of the heme-uptake system in Escherichia coli can enhance Cd(II) immobilization and facilitate Cr(VI) reduction. The ferruginous hemeprotein HhuH exhibits the ability to chelate Cd(II) and reduce Cr(VI). The presence of Cd(II) and Cr(VI) in strain SY1 initially led to Fe starvation. Subsequently, the hemeprotein HhuH facilitated Cd(II) adsorption and Cr(VI) reduction, thereby restoring normal cellular Fe homeostasis. Our findings explain the hemeprotein-mediated mechanism for Cd(II) adsorption and Cr(VI) reduction, providing further insights into the correlation between heavy metal and Fe metabolism.IMPORTANCEIron (Fe) is an indispensable trace element for many organisms, and virtually, all bacteria require Fe as a cofactor in enzymes to facilitate redox reactions involved in fundamental cellular processes during periods of heavy metal stress. Understanding bacterial response to Fe in heavy metal contamination is essential. Therefore, our study elucidates Cd(II) adsorption and Cr(VI) reduction processes mediated by the Fe-bearing hemeprotein HhuH. It is a unique trifunctional protein capable of chelating Cd(II) and reducing Cr(VI), demonstrating significant potential in the environmental remediation of heavy metals.