Abstract #52

Das Hefeperlenspiel: Life history and genomics of immobilized yeast. R Rosenzweig¹, A Kruckeberg¹, K McInnerney², B Myers³, T Taylor³. 1) Div Biol Sciences, Univ Montana, Missoula, MT; 2) Genomics Core, Montana State Univ, Bozeman, MT; 3) Biol Eng, Utah State Univ, Logan, UT.
   To date, all investigations into mechanisms that extend yeast chronological lifespan have focused on viability of non-growing planktonic cells in nutrient-depleted batch culture. Not surprisingly, screens for mutants which alter chronological lifespan have recovered genes whose activities influence resistance to prolonged and extreme caloric restriction. But yeasts largely cease to reproduce in another environmental context. Continuously or semi-continuously fed bioreactors populated by yeast immobilized in alginate beads can be maintained for weeks, producing close-to-theoretical yields of ethanol but little biomass. We sought to understand how yeast uncouple metabolism from reproduction and live for long periods in the absence of caloric restriction. Demographic and genomic parameters were estimated in nutrient non-limited log-phase, nutrient-depleted stationary-phase, nutrient-limited chemostat, and nutrient non-limited immobilized culture. Stationary-phase and immobilized populations consist largely of virgin, unbudded cells; chemostat and log-phase populations contain many budded cells and show even age distribution. Immobilized yeasts ferment glucose more rapidly than either chemostat or log-phase cells. This physiological state, where metabolic flux appears maximized at the expense of cell growth and reproduction, is supported by a unique and highly reproducible pattern of gene expression that involves significant changes in one-fifth the yeast genome. The biological function of many of these genes is poorly understood. Abundance of certain transcripts required for cell cycle progression (e.g., PCL1, MCD1) is several-fold decreased, whereas abundance of other transcripts involved in glycolysis and maintenance of glucose repression (e.g., HXT3, RGS2, TYE7) is several-fold increased. Among the genes most highly expressed in immobilized cells is MSN4, a transcription factor required for Cyr1-Ras-mediated extension of chronological lifespan.

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