Analysis of an engineered sulfate reduction pathway and cadmium precipitation on the cell surface

We previously have genetically engineered an aerobic sulfate reduction path way in Escherichia coli for the generation of hydrogen sulfide and demonstr ated the pathway's utility in the precipitation of cadmium. To engineer the pathway, the assimilatory sulfate reduction pathway was modified so that c ysteine was overproduced. Excess cysteine was then converted by cysteine de sulfhydrase to an abundance of hydrogen sulfide, which then reacted with aq ueous cadmium to form cadmium sulfide. In this study, observations of vario us E, coli clones were combined with an analysis of kinetic and transport p henomena. This analysis revealed that cysteine production is the rate-limit ing step in the engineered pathway and provided an explanation for the phen omenon of cell surface precipitation. An analytical model showed that cadmi um sulfide must form at the cell surface because the rate of cadmium sulfid e formation is extremely fast and the rate of sulfide transport is relative ly slow. (C) 2001 John Wiley & Sons, Inc.