Figure 1.

Ammonium-induced down-regulation of Gap1 is impaired in act1-1 mutant cells. (A) Cells were grown on proline medium, and Gap1 activity was assayed by measuring incorporation of [¹⁴C]citrulline (0.1 mM) before (t = 0) and at various times after addition of (NH₄⁺)₂SO₄ (10 mM) in strains NY13 (wild type; ○), NY279 (act1-1; •), and 24346c (wild type derived from Σ1278b; ▪). The Gap1 activities were calculated in nanomoles per minute per milliliter to avoid dilution effect due to NH₄⁺-triggered arrest of Gap1 synthesis. In proline-grown cells, initial Gap1 activity is lower in act1-1 cells than in isogenic wild-type cells (1.3 vs. 4.2 nmol · min⁻¹ · ml⁻¹). (B) Immunoblot of Gap1 from membrane-enriched cell fractions prepared before (t = 0) and at several times after addition of (NH₄)₂SO₄. Note that the disapearance of Gap1 signal in the NY13 wild-type strain is slower compared with the wild type derived from Σ1278b.