Identification of ferritin variants with increased cadmium selectivity by in vivo cloning and mutagenesis in Saccharomyces cerevisiae

Abstract

Uptake of heavy metals into microorganisms is an attractive bioremediation strategy, but low cost, safe and effective solutions remain lacking. Diverse organisms express ferritin, a multimeric protein that stores thousands of ferric ions and can take up other metals, with the notable exception of Saccharomyces cerevisiae. Nonetheless, this yeast has been shown to express functional ferritin subunits from soybean, increasing iron accumulation but with little impact on cadmium uptake. Compared to soybean, pea and horse ferritins bind more cadmium, pointing to amino acid residues that might be responsible for the different specificities. Chimeric soybean ferritins were constructed and analyzed in yeast. Additionally, random mutagenesis and screening were applied to identify another variant that accumulate more cadmium. Although expression was similar for each variant, cadmium accumulation varied. Importantly, tested in a model for bioremediation, culturing the ferritin-expressing strains in media containing 100 μM CdCl2 led to 80 % removal of the metal.