| The phenomenon of life span extension by restricting the caloric intake of an organism has been known and studied for decades. Originally observed in rats, the study of caloric restriction (CR) has expanded to include not only mammals but invertebrates such as D. melanogaster, C. elegans , as well as single celled organisms such as S. cerevisiae . The genetic tractability and short life spans of these organisms, in particular yeast, has enabled us and others to identify the genetic components involved in life span extension during CR and other mild stresses.;This dissertation elucidates a genetic pathway involved in extending the replicative life span of yeast in response to CR. We originally showed that genes encoding components of the NAD+ salvage pathway, such as NPT1 and PNC1, are required for replicative life span extension via multiple stresses, such as mild heat stress and CR. The nicotinamidase Pnc1 is required to catalyze the conversion of nicotinamide, a potent physiologic inhibitor of Sir2, to nicotinic acid. Sir2, an NAD+ dependent histone deacetylases (HDACs), catalyzes the formation of hetero-chromatin at multiple loci, while releasing nicotinamide and O-acetyl-ADP-ribose as by-products of the reaction. The formation of hetero-chromatin at the yeast RDN1 locus by Sir2 inhibits the formation of extra-chromosomal rDNA circles (ERCs), a causative agent of aging in yeast. Our observations demonstrated that Pnc1 is the only factor within the NAD+ salvage pathway that is upregulated in response to stresses that extend lifespan. We show that the expression of Pnc1 during CR requires the presence of the general stress response transcription factors, Msn2/4. Furthermore, we go on to show that the regulation of replicative life span during CR involves not just Sir2, but the activity of the Sir2 homolog Hst2 as well. These observations have enabled us to construct a model in which environmental stimuli regulate the life span of yeast through Msn2/4 mediated induction of PNC1. |
PDF Full Text Request