Cellular responses to L-serine in Saccharomyces cerevisiae: roles of general amino acid control, compartmentalization, and aspartate synthesis.

In addition to its other roles, L-serine functions in one-carbon metabolism and is interconvertable with glycine via serine hydroxymethyltransferases. However, the transcriptional response by Saccharomyces cerevisiae to L-serine addition is markedly different from that to glycine, with L-serine acting as a nutrient source rather than one-carbon units. Following addition of excess L-serine, 743 genes showed significant expression changes. Induced functions included amino acid synthesis, some stress responses, and FeS metabolism, while ribosomal RNA processing, ribosome biogenesis and hexose transport were repressed. A co-regulated network of ten transcription factors could together control more than 90% of the induced and repressed genes forming a general response to changes induced by other amino acids or stresses and including the general amino acid control system usually activated in response to starvation for amino acids. A specific response to L-serine was induction of CHA1 encoding serine (threonine) dehydratase. L-serine addition resulted in a substantial transient increase in L-aspartate, which is, rather than L-glutamate, the major metabolite for short-term storage of ammonia derived from degradation of L-serine. L-aspartate synthesis was exclusively through mitochondrial metabolism of L-serine to pyruvate and ammonia, involving Cha1p, cytoplasmic pyruvate carboxylases Pyc1p and Pyc2p, and the cytoplasmic aspartate aminotransferase Aat2p.