Identification of genes responsible for reducing palladium ion in Escherichia coli

Palladium (Pd) is commonly used as a catalyst for automobiles and electronic devices, and a reliable source of Pd is required for continued commercial applications. Biomineralization has attracted attention as an inexpensive and eco-friendly recycling approach for a continued supply of Pd. Escherichia coli is one of the best hosts for collecting Pd because it grows rapidly and requires an inexpensive minimal medium. Although E. coli can reduce Pd ions, the mechanism of reduction has not been thoroughly investigated. In this study, we investigated the genes involved in the reduction of Pd ions in E. coli. A gene responsible for the reduction of Pd ions was identified from approximately 4000 genes, other than essential genes, by using the single-gene-knockout library. The rate of reducing Pd ions by E. coli cells was evaluated. Among the investigated single-gene-knockout strains, 7 strains including the gene related to membrane transport, transcriptional regulation, and metabolic enzyme promote the reduction of Pd ions, and 73 strains including the genes related to formate metabolism and molybdopterin synthesis repress the reduction of Pd ions. Our results may provide a new perspective for the improvement of the bioreduction of minor metals.