Enhanced arsenate uptake in Saccharomyces cerevisiae overexpressing the Pho84 phosphate transporter

Arsenate is a major toxic constituent in arsenic-contaminated water supplies. Saccharomyces cerevisiae was engineered as a potential biosorbent for enhanced arsenate accumulation. The phosphate transporter, Pho84p, known to import arsenate, was overexpressed using a 2μ-based vector carrying PHO84 under the control of the late-phase ADH2 promoter. Arsenate uptake was then evaluated using a resting cell system. In buffer solutions containing high arsenate concentrations (12,000 and 30,000 ppb), the engineered strains internalized up to 750 μg of arsenate per gram of cells, a 50% improvement over control strains. Increasing the cell mass 2.5-fold yielded a proportional increase in the volumetric arsenate uptake, while maintaining the same level of specific uptake. At high levels of arsenate, loss from the intact cells to the medium was observed with time; knockouts of two known arsenic extrusion genes, ACR3 and FPS1, did not prevent this loss. At trace level concentrations (120 ppb), rapid and total arsenate removal was observed. The presence of 50 μM phosphate reduced uptake by approximately 15% in buffer containing 80 μM (6,000 ppb) arsenate. At trace levels of arsenate (70 ppb), the phosphate reduced the initial rate of uptake, but not the total amount removed. PHO84 mRNA levels were nearly 30 times higher in the engineered strains relative to the control strains. Uptake may no longer be a limiting factor in the engineered system and further increases should be possible by upregulating the downstream reduction and sequestration pathways.