| Abstract: | The presence of low levels (2–4 mM) of (NH4)2SO4 in sporulation medium results in a complete inhibition of ascus formation in Saccharomyces cerevisiae. Tests with other utilizable nitrogen sources and with the non-metabolizable ammonia analog, methylamine, indicate that the primary inhibitory effect is exerted by ammonium (NH4+) and not a metabolite of NH4+. Viability is also reduced but to a much lesser extent than sporulation. The levels of NH4+ that repress sporulation do not affect the rate of oxygen utilization or ATP levels of cells in sporulation medium. The period during which the cells are sensitive to NH4+ is relatively long, and they do not escape inhibition until the meiotic commitment stage. The most sensitive period is roughly coincident with the end of premeiotic DNA synthesis. NH4+ does not inhibit the initiation of premeiotic DNA synthesis, but DNA replication is arrested after initiation and continued incubation in the presence of NH4+ leads to massive DNA degradation. Commitment to recombination is not affected specifically by NH4+. Preliminary experiments with an NH4+-resistant mutant indicates that levels of NH4+ sensitivity are under genetic control. |
