Catalase-peroxidase StKatG is a bacterial manganese oxidase from endophytic Salinicola tamaricis

Manganese (Mn) oxides in iron/manganese plaques are widely distributed in the rhizosphere of wetland plants and contribute significantly to elemental cycling and pollutant removal. Mn oxides are primarily produced by bacterial processes using Mn oxidases. However, the molecular mechanism underlying the formation of rhizosphere Mn oxides is still largely unknown. This study identified a manganese-oxidizing enzyme, the catalase-peroxidase StKatG, from an endophytic bacterium Salinicola tamaricis from the wetland plant. The gene encoding StKatG was cloned and overexpressed in Escherichia coli. The recombinant StKatG displayed different structure and enzymatic properties from the previously reported Mn oxidases. The enzyme activity of StKatG yielded Mn oxides with the mixed-valent state: Mn(II), Mn(III), and Mn(IV). The optimum pH and temperature for StKatG are 7.5 and 50 °C, respectively. Structurally, StKatG is organized into two domains, whereas the reported Mn oxidases are mainly single-domain proteins. Based on the site-directed mutagenesis studies, the presence of aspartic acid (Asp) residues in the loop of StKatG are critical to Mn-oxidizing activity. These findings identified a novel bacterial Mn oxidase and provided insights into the molecular mechanism of Mn oxidation in the plant rhizosphere.