FIG. 5.

Cell density-dependent modes of the response to nitrogen starvation. Cells (2 × 10⁷/ml and 10-fold serial dilutions of wild-type and Δfnx1 cells) were incubated for 6 days in medium lacking nitrogen and spotted onto the top row of the plates with 10-fold serial dilutions of each independent culture spotted underneath. (A) Wild-type cells after heat shock treatment. Only the culture at 2 × 10⁷ cells/ml was able to survive. (B) Wild-type cells not subjected to heat shock. The smaller size of the colonies at the cell density of 2 × 10⁷ cells/ml reflects the longer recovery time after starvation. (C) Δfnx1 cells not subjected to heat shock. The culture at 2 × 10⁷ cells/ml contained no viable cells. (D) Mixture of equal numbers of wild-type ura4-D18 (ura⁻) and Δfnx1 (ura⁺) cells not subjected to heat shock. Only Δfnx1 cells are able to grow on the EMM (without uracil) plate used for determination of viability. The presence of fnx1⁺ cells did not rescue the loss of viability of fnx1 null cells.