Figure 4.

(A) Efflux of ³⁵SO₄⁼ induced by addition of extracellular K₂SO₄ to ³⁵SO₄⁼-loaded met3Δ cells. Cells were grown and S-starved 2 h as in Fig. 3. At t = 0, 10 μM [³⁵S] K₂SO₄ was added to a 1% suspension in APG, and aliquots were centrifuged at the indicated times and the ³⁵S in the supernatant measured (▴). For aliquots represented by ●, 0.1 mM unlabeled K₂SO₄ was added 1 min before centrifugation. Addition of K₂SO₄ results in the 1 min loss of 20% of the cellular ³⁵S for the first addition (t = 7 min) and 15% of the ³⁵S for the last addition (t = 53 min). (B) Transport activity in the same cell preparations as in A, except that 10 μM unlabeled K₂SO₄ was added at t = 0. At the indicated times, aliquots were withdrawn for measurement of 0.5 min influx of ³⁵SO₄⁼ (0.1 mM extracellular [SO₄⁼]).