HEAT STRESS fUNCTION OF S.POMBE

A Novel Heat Stress Function of S.Pombe



LITERATURE REVIEW

Yeast are simple fungi. The term "yeast" refers more to a life-style than to a phylogenetic classification. Yeast refers to the unicellular phase of the life cycles of many different fungi, but it is used more commonly as a generic term for fungi that have only a unicellular phase. The organisms most often called "yeast" such as common baking or brewing yeast, are strains of the species Saccharomyces cerevisiae (Fowell 1969a). As fungi, they are classified as ascomycetes, a group which also includes a number of other popular genetic organisms, such as Neurospora and Sordaria (Fincham & Day 1971). Except when we refer to other species of yeast by name, we will use the term "yeast" to refer to Saccharomyces cerevisiae.

Yeast have simple nutritional needs. Unable to carry out photosynthesis, they require a reduced carbon source which can be as simple a compound as acetate. In addition, they also require a nitrogen source such as ammonium sulfate. Yeasts can use a variety of organic nitrogen compounds, including urea and various amino acids. The only other complex compound that they require is the vitamin, biotin. Of course, they also require a variety of salts and trace elements.

Another characteristic of most yeast, including S. cerevisiae, is that they divide by budding, rather than by binary fission (Byers 1981). A small bud emerges from the surface of the parent cell and enlarges until it is almost the size of the parent. (Tepass and Knust, 1993; Tanentzapf and Tepass, 2003).

People have used yeast, undoubtedly one of the earliest domesticated organisms, for controlled fermentation of food and drink and for leavening in baking throughout recorded history. Today, they are also used in a variety of commercial fermentation and biomass conversion processes. Their usefulness is based on their ability to convert sugars and other carbon sources into ethanol in the absence of air (anaerobic), and into carbon dioxide and water in the presence of air (aerobic). Ethanol is a valuable alternative to petroleum as a fuel and as a raw material for the manufacturing of many important commercial chemicals. Yeast is also good food. It is rich in protein and is an uncommonly good source of the B vitamins. It provides a valuable source of nutrients that are important in low-meat or vegetarian diets. But while few emanations from the kitchen are quite as tantalizing as the yeasty aroma of baking bread, most people agree that pure yeast tastes pretty bad.

Both haploid and diploid yeast cells divide by budding (see Figure 2). The cell division cycle begins with a single, unbudded cell (Pringle & Hartwell 1981; Byers 1981). This cell buds, the bud grows to nearly the size of the parent cell, the nucleus divides, and the two cells separate into two unbudded cells. The cycle then begins again for both of the cells. The result is an exponential increase in the number of cells with a doubling time equal to the mean cell-division-cycle ...