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Yeasts can be divided into two metabolic groups: facultative anaerobes and obligate aerobes. The facultative anaerobes are capable of anaerobic growth and fermentative conversion of sugars to ethyl alcohol, CO2, and cell mass, in addition to the aerobic conversion of sugars to CO2 and H2O, and much higher yields of cell mass. Fermentative yeasts such as S. cerevisiae (Figure 1.1), when grown in the presence of 3 ppm of the DNA intercol-lating agent acriflavine, can have their mitochondrial DNA selectively mutated so that mitochondria are eliminated, resulting in obligately fermentative strains unable to utilize oxygen (3). Such strains produce smaller cells than wild-type strains and result in "petite" colonies that are notably reduced in size.

Baker's yeast was originally obtained from the brewing industry; the top yeast S. cerevisiae was conveniently skimmed from the top surface of fermentation tanks. During the mid-1800s the brewing industry converted to strains of the bottom-settling yeast Saccharomyces carlesburgensis, which precipitated the establishment of the baker's yeast industry. Producing baker's yeast using sucrose derived from molasses requires vigorous aeration of the culture medium so that a maximum amount of carbon flows to cell mass production and not to ethyl alcohol formation. Vigorous aeration of S. cerevisiae strains in the presence of an abundant level of carbohydrate (about 3%) results in the metabolic dominance of fermentation and is known as the crabtree effect (4). This in turn results in a significant level of ethyl alcohol and a notably reduced level of cell mass. The baker's yeast industry is able to overcome the crabtree effect using incremental feeding which involves the pulsed addition of molasses to aerated culture tanks, so that at no time does the residual level of sucrose rise above 0.0001%. Thus there is no feedback repression of mitochondria formation caused by elevated levels of sucrose. In this case, derepressed mutants that do not exhibit feedback repression are not used. The yeast Candida utilis is facultatively anaerobic; however, under conditions of vigorous aeration and elevated sugar levels the crabtree effect is not observed. Thus, the organism can be conveniently used to convert the lactose in whey and the sugars in sulfite waste liquor to cell mass for use as food and fodder yeast.

All yeasts are capable of utilizing glucose. The utilization of other sugars depends on the species; the spectrum of sugars used constitutes a major criterion for the identity of a©

Conjugation In haploid strains

Growth in high

2N diploid 2N diploid

acetate agar T

Ascus with 4 ascospores

Ascus with 4 ascospores

Intraascus conjugation

Intraascus conjugation

Figure 1.1 Life cycle of Saccharomyces cerevisiae.

t Growth in high glucose medium yeasts. All yeasts are capable of utilizing ammonium sulfate as a sole source of nitrogen. Very few yeasts are capable of utilizing nitrate as a sole nitrogen source. Among asco-spore-producing yeasts, the number (1, 4, or 8) and shape of ascospores (spherical, oval, kidney, hat, saturn, needle) in asci constitutes an additional major criterion for genus and species identity. Most yeasts divide by budding; however, members of the strongly fermentative yeast genus Schizosaccharomyces divide solely by transverse fission (Figure 1.2).

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