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Q10 - Yeast Prion [GAR+] and Bacterial Metabolite Signaling

Theoretical 1 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Gene Regulation: Eukaryotic & Epigenetics

Yeast (Saccharomyces cerevisiae) prion protein Pma1 is found in two significantly different conformations.

In one of these conformations, the protein clumps resulting in loss of physiological function of the protein. This results in the [GAR+] phenotype. This phenotype is spread by cytoplasmic, non-Mendelian inheritance: sporadically daughter cells are formed which do not contain Pma1 protein clumps, so these have a [gar−] phenotype.

Catabolic repression describes the phenomenon of yeast using glucose as a sole source of carbon and energy, despite the presence of other organic compounds that yeast cells can use in metabolism. Only in the absence of glucose will yeast cells use other sources of carbon and energy, e.g. glycerol.

In [GAR+] yeast, there is no catabolic repression.

Lactic acid secreted by Staphylococcus gallinarum bacteria, which live alongside S. cerevisiae, can induce a change in the yeast phenotype from [gar−] to [GAR+]. In an experiment, yeast was grown on solid agar-agar minimal medium containing glycerol (Gly) or glycerol plus glucosamine (Gly+GlcN). Glucosamine induces catabolic repression in yeast but is not metabolized by S. cerevisiae.

Panel A shows yeast colony growth on Gly and Gly+GlcN plates for conditions X (serial dilution control), Y (yeast only), F (yeast with S. gallinarum), C (S. gallinarum only), D (yeast with filtered unused bacterial medium), and E (yeast with filtered post-cultivation bacterial medium). Panel B shows the fraction of GAR+ colonies increasing with lactic acid concentration. Panel A. Colony growth under different conditions. X = serial dilution of S. cerevisiae, Y = S. cerevisiae only, F = S. cerevisiae with S. gallinarum, C = S. gallinarum only, D = S. cerevisiae + filtered unused bacterial medium, E = S. cerevisiae + filtered medium after S. gallinarum cultivation.

Bar graph showing fraction of GAR+ colonies on the y-axis versus lactic acid concentration on the x-axis. The fraction of GAR+ colonies increases with higher lactic acid concentrations. Panel B. Fraction of [GAR+] colonies in the yeast population at increasing lactic acid concentrations.

Q10.1. A point mutation in the gene encoding the Pma1 protein, without any major conformational change, will result in the [GAR+] phenotype.
Q10.2. The occurrence of [gar-] cells is higher in cultures lacking lactic acid, than cultures containing lactic acid.
Q10.3. Cytoduction (the transfer of cytoplasm) from a [GAR+] cell to a [gar-] cell will change its phenotype to [GAR+].
Q10.4. The results show that the biggest change in yeast phenotype is caused by the products of the bacteria themselves, rather than components of the bacterial medium.
Q10.5. Lactic acid secreted by S. gallinarum diffuses in the agar-agar solidified medium.
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Question reproduced from IBO 2023, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF