Yeast Introduction

Yeasts are eukaryotic, single-celled microorganisms around 3–40 µm in diameter. A single yeast cell is too small to be seen with the naked eye, but these organisms are indeed omnipresent. Yeast can live in humans and are present in various environments throughout the world - on plants, animals, soils, and oceans. There are over 1,500 known species of yeast. Yeasts are thought to have originated hundreds of millions of years ago, and may have evolved from multicellular ancestors.

 

S. Cerevisiae Introduction

Saccharomyces cerevisiae (also called budding yeast) is round and around 5–10 μm in diameter. S. cerevisiae converts carbohydrates to carbon dioxide and alcohols through a process called fermentation. The products of this reaction have been used in baking and alcohol production for thousands of years, predating recorded history. The desire to cultivate wheat to ferment into bread may have motivated humans to adopt an agricultural lifestyle.

 

S. cerevisiae is not airborne, requiring a vector to spread. Ancient people used S. cerevisiae in things like the skin of grapes for fermentation without knowing what was performing the reaction. Anton van Leeuwenhoek, who discovered microorganisms in the 1670s, identified S. cerevisiae in 1680, although he thought it was not alive. Pasteur connected S. cerevisiae to fermentation in 1856. S. cerevisiae became a model organism for studying aging in the 1960s. S. cerevisiae has been found in the human gut microbiome as well as living on human skin. 

 

S. Cerevisiae Genome

S. cerevisiae cells can exist as both a diploid cell with 16 chromosome pairs or as a haploid cell with 16 chromosomes. These 16 chromosomes contain about 12 million base pairs, 6,275 genes, and 250 introns. Only about 5,800 of these genes correspond to open reading frames and are believed to be functional, and about 30% of these functional genes have homologs in humans.

 

S. Cerevisiae Mitosis and Meiosis

Chromosome III contains one of two alleles of a mating-type locus called MAT - a MATa or a MATα locus. This locus is only expressed in the haploid cells and determines their mating type - either a or α. Both haploid and diploid yeast cells reproduce by mitosis, referred to as budding. 

 

Diploid cells can additionally undergo meiosis to produce four haploid spores: two a spores and two α spores. Meiosis usually only occurs in response to adverse conditions, such as nutrient deprivation. Ideal conditions for a yeast cell include a temperature of about 30 °C. Yeast spores can exist in a dormant state for a long amount of time waiting for better conditions (how long?). 

 

Under suitable conditions, spores can become metabolically active haploid cells which can reproduce by budding and are capable of mating with other haploid cells of the opposite mating type to produce a stable diploid cell. 

 

Defining Yeast Aging

Budding in S. cerevisiae is asymmetric. The mother cell retains most of the misfolded proteins that have accumulated, while the bud daughter cell inherits mostly well-formed proteins, most of which are the proteins newly created during mitosis. One notable exception is the spindle pole body (SPB) of the budding yeast, which is its centrosome equivalent. The SPB duplicates in a conservative manner, producing an old and a new SPB. The old SPB is usually inherited by the bud (McMurray 2009).

 

The number of times an S. cerevisiae cell has budded is referred to as its replicative lifespan, which is counted in generations. Each budding generation increases the likelihood of the yeast cell’s death, which is referred to as replicative aging. Notable hallmarks of yeast replicative aging are an increased time between budding generations and a budding scar on the yeast cell wall. Budding usually does not happen at the same place twice, perhaps because of the previous generation’s budding scar. Yeast cells do not lose telomeres from budding. The mechanisms of asymmetric budding break down as the yeast cell ages replicatively and the division becomes more symmetric (K.A. Steinkraus 2009). After about twenty divisions (source needed), the S. cerevisiae cell exits the normal cell cycle and enters into a state of replicative senescence. 

 

S. cerevisiae replicative senescence was discovered in 1959 (Mortimer and Johnston ‘59) but was not commonly known until about a decade later (need evidence) and this is when they become a model organism for the study of aging. Yeast replicative senescence actually predates Haylick’s famous 1961 discovery that non-stem human somatic cells also do not divide indefinitely, and enter into a state of senescence. Hayflick’s 1961 paper does not cite Mortimer and Johnston’s paper. 

 

The chronological lifespan of a yeast cell is the length of time that the yeast cell survives without dividing. Nutrient exhaustion is one method of increasing chronological lifespan in yeast cells. There have been numerous interventions (discussed below) that have increased the maximal replicative lifespan of yeast cells. 

 

ERCs

As S. cerevisiae cells age, rDNA circles arise independently of the chromatin rDNA. These extrachromosomal rDNA circles (ERCs) replicate with each subsequent division and in general accumulate in the mother cell and are not inherited by the daughter cell. Hence, old and especially senescent yeast cells are characterized by enlarged and fragmented nuclei (Sinclair 1997 ). 

 

Yeast Nucleolus

The nucleolus of S. cerevisiae is composed mainly of 100–200 copies of rDNA in tandem on chromosome XII. Unlike DNA replication in general, rDNA is unique in yeast in that replication is unidirectional. Yeast have cellular machinery to block the replication machinery in one direction in the nucleolus. The FOB1 protein is required for this blocking, as FOB1 negative mutants have bidirectional rDNA replication. HRM is a class of mutations in yeast that affect recombination in the rDNA and not other DNA. Hrm1 mutants enjoy a much longer life span. The cloning of HRM1 reveals that it is identical to FOB1. Induction of ERCs reduces the lifespan of yeast.