Metabolic engineering of an aerobic sulfate reduction pathway and its application to precipitation of cadmium on the cell surface

The conversion of sulfate to an excess of free sulfide requires stringent r eductive conditions. Dissimilatory sulfate reduction is used in nature by s ulfate-reducing bacteria for respiration and results in the conversion of s ulfate to sulfide. However, this dissimilatory sulfate reduction pathway is inhibited by oxygen and is thus limited to anaerobic environments. As an a lternative, we have metabolically engineered a novel aerobic sulfate reduct ion pathway for the secretion of sulfides. The assimilatory sulfate reducti on pathway was redirected to overproduce cysteine, and excess cysteine was converted to sulfide by cysteine desulfhydrase. As a potential application for this pathway, a bacterium was engineered with this pathway and was used to aerobically precipitate cadmium as cadmium sulfide, which was deposited on the cell surface. To maximize sulfide production and cadmium precipitat ion, the production of cysteine desulfhydrase was modulated to achieve an o ptimal balance between the production and degradation of cysteine.