Ammonium Ion Repression of Sporulation in Saccharomyces cerevisiae

The role of NH,+ in the regulation of sporulation in Saccharomyces cerevisiae has been studied by analysing the effects of NH,+ and methylammonium ions on sporulation in a wild-type strain; a strain homozygous for the spdl mutation, (derepressed sporulation on a number of carbon sources, reduced sensitivity to NH,+-inhibition of sporulation) ; and a strain homozygous for the gdhA6 mutation, (the structural gene for the anabolic NADP+-dependent glutamate dehydrogenase). The addition of ammonium or methylammonium ions to sporulation medium resulted in incomplete ascus formation. The gdhA6 mutation resulted in a loss of sensitivity to NH,+-inhibition of initiation of sporulation. At higher concentrations of NH,+ the strain homozygous for the gdhA6 mutation was even less sensitive than the sporulation-derepressed strain. However, in the formation of complete asci all three strains behaved very similarly over the whole range of ammonium sulphate concentration. These studies indicate there are at least two separate stages affected by NH:; one early, possibly initiation, the other later, concerned with the organization and delimitation of mature spores. From the reduced sensitivity to NH,+-inhibition of initiation of sporulation conferred by the gdhA6 mutation, and the fact that similar results were obtained using methylammonium ion, it is concluded that some metabolite of NH,+ is responsible rather than NH,+ itself. In addition, the data provide some insight into the nature of the spdl mutation