The long-term objective of the proposed study is the elucidation of the mechanism responsible for the regulation of the expression of genes in Saccharomyces cerevisiae in response to the availability of nitrogen. The results of this study should contribute to the general understanding of mechanisms of gene regulation in eukaryotic cells. In particular, the interaction of the activator, the product of GLN3, with it targets, DNA sequences located upstream of the transcriptional starts of the regulated genes GLN1, the structural gene for glutamine synthetase; GDH2, for NAD+-linked glutamate dehydrogenase; and of GAP1, for the general amino acid permease will be explored by physical and chemical methods. Furthermore, the modification of the GLN3 product and of glutamine synthetase by the product of the regulatory gene URE2 in response to glutamine will be studied by genetic and biochemical methods. Finally, additional regulatory proteins that cooperate with the GLN3 product or, in the case of the regulation of GAP1 expression, replace the GLN3 product will be isolated and studied.