Abstract

The glycine cleavage system (GCS) is a multienzyme complex containing four proteins. The GCS, which is important for the growth and viability of organisms ranging from bacteria to humans, catalyses the oxidative cleavage of glycine into CO 2 and NH 3 . Concomitantly it generates the C1-donor 5,10-methylenetetrahydrofolate and the electron donor NADH. NH 3 is an important precursor for cellular nitrogen metabolism. The C1-donor 5,10-methylenetetrahydrofolate is a precursor for the biosynthesis of C1-end products such as adenine, thymidylate, serine and methionine. The goal of my research was to study the regulatory mechanisms controlling GCS activity. The expression of GCV3 , a yeast gene that codes for one of the four GCS subunits, was analyzed in detail. This revealed that GCV3 expression is regulated by the availability of glycine, and cellular demand for the metabolic products of glycine cleavage. 10 mM glycine in minimal medium (SD) induced GCV3 expression about 4-fold. Supplementing with the C1-metabolic end products repressed GCV3 expression about 3-fold. Both glycine induction and repression by the C1-end products were found to be Bas1p-dependent. The upstream promoter elements required for regulation by Bas1p were localized. Expression of GCV3 is also subject to the general control system in a Gcn4p-dependent fashion. The elements utilized by Gcn4p have been characterized. A GATAA sequence located at -167bp upstream of the start codon is used by the nitrogen regulation system. Gcr1p, a transcription activator for glycolytic genes, is involved in regulating GCV3 in the presence of glucose. Evidences are also presented for an as yet unidentified regulator that represses expression in SD. Additional results presented in this thesis suggest that Rap1p, Nil1p, Ure2p and Deb1p also regulate GCV3