FIG. 4.

(A) Δnrd1 cells are indistinguishable from wild-type cells in their proliferation ability in rich and poor medium. The h⁻ (open symbols) and h⁻ Δnrd1 (solid symbols) cells were grown in PM medium to mid-log phase. Each strain was then inoculated into PM medium containing 0.5% NH₄Cl and 2% glucose (circles), or PM medium containing only 0.1% NH₄Cl and 0.5% glucose (squares) at 10⁶ cells/ml and incubated at 30°C for the indicated times, and the number of cells was counted. (B) Δnrd1 cells mate without nitrogen starvation. The h⁹⁰ nrd1⁺ and h⁹⁰ Δnrd1 cells were grown in PM medium to mid-log phase. Each strain was then inoculated in PM medium or PM medium containing 0.5% glucose at 3 × 10⁶ cells/ml and incubated at 30°C for the indicated times, and the number of zygotes formed was counted. The percent population of zygotes was calculated as described in Materials and Methods. (C) Cell morphology of Δnrd1 cells. Each strain was incubated in 0.5% glucose medium with or without nitrogen at 30°C for 24 h. (D) Effect of osmotic pressure on enhanced conjugation. The h⁹⁰ Δnrd1 cells were grown in complete PM medium and then incubated in PM medium containing only 0.5% glucose or 0.5% glucose plus 1.5% sorbitol for the indicated times, and the number of zygotes formed was counted. (E) Conjugation of Δnrd1 cells at various nitrogen concentrations. The h⁹⁰ and h⁹⁰ Δnrd1 cells were grown in PM medium to mid-log phase. Each strain (5 × 10⁶ cells) was inoculated into 1 ml of PM medium containing 0.5% glucose and the indicated concentrations of nitrogen and incubated at 27°C for 40 h, and the number of zygotes was counted. (F and G) Conjugation of Δnrd1 cells in nitrogen-starved, high-glucose medium. The h⁹⁰ and h⁹⁰ Δnrd1 cells were grown in PM medium to mid-log phase. Each strain was then inoculated in NH₄Cl-free PM medium (F) or PM medium containing 0.01% NH₄Cl (G) at a concentration of 1 × 10⁷ and 5 × 10⁶ cells/ml, respectively, and incubated at 30°C for the indicated times, and the number of zygotes was counted. (H) Overexpression of nrd1⁺ inhibits the conjugation of wild-type cells. Exponentially growing h⁹⁰ leu1-32 cells harboring pALSK or pALSKnrd1⁺ were incubated for the indicated times in nitrogen-free PM medium containing 0.5% glucose, and the number of zygotes was counted.