FIG 1.

Ssu1 and Ssu2 are involved in sulfite sensitivity and nitrate efflux. (A) Deletion of the SSU1 or SSU2 gene produces sulfite sensitivity. The ssu1Δ and ssu2Δ strains were grown in YPD. Serial 10-fold dilutions were spotted on pH 3.5 buffered synthetic medium containing 5 mM ammonium chloride plus sodium sulfite at the concentration indicated. Plates were incubated at 37°C for 2 days. (B) The ssu2Δ strain accumulates nitrate. Ammonium-grown cells were resuspended in synthetic medium at an OD₆₆₀ of 2 to 3 and then nitrogen starved for 120 min. Nitrate accumulation assays were triggered with 1 mM nitrate. Intracellular nitrate was determined in ethanolic cell extracts. (C) Net nitrate uptake increases in the ssu2Δ ynrΔ strain. Ammonium-grown cells were resuspended to an OD₆₆₀ of 10 in nitrogen-free medium buffered at pH 5.5 for 60 min. Nitrate uptake assays were triggered with 0.1 mM nitrate. Nitrate uptake was determined as extracellular nitrate depletion for 60 min. Data ± SE from three independent experiments are shown.