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Table 4.6 Contd.

Enzyme

Location

Comments

Reference

NAD(P)-aldehyde dehydrogenase

Mn-superoxide dismutase

Cytochrome c oxidase

Adenylate kinase

FrATPase

Succinate dehydrogenase NADH-ferricyanide reductase Succinate-ferricyanide reductase Succinate-cytochrome c reductase Succinate oxidase NADH oxidase Cytochrome c oxidase

Fumarase

TCA cycle enzymes

Hydroxymethyl-CoA synthetaseglutaryl Hudroxymethyl-CoA reductaseglutaryl

NADH kinase

Proline oxidase

NADH-ubiquinone-6 oxidoreductase

Pyruvate dehydrogenase

Carb oxypeptidase Aminopeptidase Dipeptidyl aminopeptidase a-Mannosidase

Phosphohydrolase

ATP-ase

Acid trehalase

Mitochondria

Mitochondria

Mitochondria - inn membrane

Mitochondria and cytosol

Mitochondria

Mitochondrial matrix and cytosol

Mitochondrial matrix

Mitochondria

Mitochondria (inner membrane)

Mitochondria

Mitochondria

Mitochondria Vacuole

Vacuole Vacuole

Vacuolar membrane Vacuole

Promitochondrial activity: 4-fold reduction

2-fold reduction

3-fold reduction absent absent

375-fold reduction 550-fold reduction

2-oxoglutadehydrogenase absent under anaerobiosis/repressed conditions (Wales et al., 1980)

Initial steps of squalene synthesis in sterolgenesis

Involved in formation of NADPH from NADH

Requires aerobiosis

Functions in place of malate/aspartate shuttle in S. cerevisiae

Marker of vacuolar membrane lacobsen and Bernofsky (1974)

Wright et al. (1995)

Bandlow et al. (1988)

Criddle and Schatz (1969)

Boonyarat and Doonan (1988)

Robinson and Srere (1985)

Coolbear and Threfall (1989)

Iwahashi and Nakamura (1989)

Brandriss (1983)

Rendueles and Wolf (1988)

Yoshihisa et al. (1988)

Lichko and

Okorokov (1990)

Okorokov and Lichko (1983)

Mittenbühler and Hölzer (1988)

tional levels together with the myriad of seemingly different criteria for inclusion. In the case of brewing yeast it is a member of the fungi kingdom which has been estimated to consist of 100000 species (Anonymous, 1996b). The 'first cut' of classification ('division') graphically demonstrates the diversity of the fungal kingdom (Table 4.7) ranging from brewing yeast, edible mushrooms and Penicillium through to 'thrush' (Candida albicans) and a host of moulds, mildews and rusts. The complexity of classification through descending hierarchy of taxa can been seen (Fig. 4.2) through tracing the lineage of brewers' yeast - S. cerevisiae - down through the kingdom to the yeast species.

For most, the practical fruits of classification are at the levels of genus, species and strain. Definitions are difficult, debatable and, occasionally controversial. For simplicity, 'popular' definitions are presented here which, although not precise, provide a 'feel' of the scope of each taxa. A genus is a 'group of species closely related in

Table 4.7 Fungal taxonomy.

Fungal division Comments

Examples

Oomycota Frequently resemble algae, often parasitic to plants

Zygomycota Filamentous, often insect parasites

Ascomycota Sac fungi, yeasts with endogenous spores, usually hyphal

Basidiomycota Yeast, exogenous spores, some plant parasites, some saprophytic

Deuteromycota Fungi imperfecti, mixed group, do not produce spores

Water mould, white rust, downy mildew

Black bread mould

Saccharomyces, powdery mildew, red bread mould (Neurospora)

Corn smut, edible mushroom, sulphur fungus, black stem rust, puffball,

Cryptococcus

Thrush (Candida albicans), Aspergillus, Bretanomyces, Verticilium, Fusarium, Penicillium

Fig. 4.2 The taxonomic hierarchy.

structure and evolutionary origin' whereas species can be defined as 'individuals, which interbreed but are unable to breed with other such groups' (Anonymous, 1996b). The final level of classification is at the level of strains. Here the phenotypic and genotypic differences between strains within a species are - taxonomically -comparatively minor.

As noted above, brewing yeast is of the genus 'Saccharomyces' and the species 'cerevisiae' resulting in the specific name S. cerevisiae (the species is always coupled with the genus and has a lower case initial letter). The names of strains of S. cerevisiae often relate to the brands they produce, brewery they originate from or, if from a culture collection, are simply described numerically.

The changing complexities of yeast taxonomy have been reviewed at length in recent years (Barnett, 1992; Campbell, 1996). Perhaps not surprisingly, the classification of yeasts has provoked much lively debate with phases of taxonomic expansion and reduction. This is ably demonstrated by considering the number of yeast genera and species reported over the years in The Yeasts, A Taxonomic Study. Between the second (Lodder, 1970) and third (Kreger-van Rij, 1984) editions of this major text, the number of genera increased from 39 to 60 and the number of species from 349 to 500. In the fourth edition (Kurtzman & Fell, 1998) there are now 100 genera representing over 700 species. This, as noted by Walker (1998) 'represents only a fraction of the yeast biodiversity on this planet'.

The genus Saccharomyces Meyan ex Rees has been subject to both contraction and slight expansion. The 41 species identified in 1970 (Lodder, 1970) were reduced to ten in 1990 (Barnett et al., 1990; Barnett, 1992a) and increased to 14 in 1998 (Kurtzman & Fell, 1998) (Fig. 4.3). The ten 'core' species described by Barnett (1992a) have been subdivided into three groups based on ribosomal RNA sequence (Vaughan-Martini & Martini, 1993) and the size and stability (see Section 4.3.2.7) of the mitochondrial genome (Piskur et al., 1998). The Saccharomyces sensu stricto group consists of S. cerevisiae and three other closely related 'sibling' species S. bayanus, S. paradoxus and

Fig. 4.3 The genus Saccharomyces Meyen ex Rees. The numbers in brackets are the molar percentage of guanine + cytosine in nuclear DNA (see Section 4.2.1.1).

S. pastorianus. Although closely related, the four species split into two 'clusters' (Table 4.8) (Montrocher et al., 1998) consisting of (i) S. bayanus and S. pastorianus and (ii) S. cerevisiae and S. paradoxus. The two clusters differ fundamentally in their response to temperature. Compared to the 'cerevisiae' cluster, the 'bayanus' cluster grows at a lower optimum and maximum temperature (see Section 4.2.3), is able utilise melibiose (Naumov, 1996) and transports fructose via an active proton sym-port (Rodrigues de Sousa et al., 1995). Despite these differences, it is worth noting that although separated into clusters, the yeasts within the Saccharomyces sensu stricto group consist of 'very closely related yeasts when the whole genus Saccharomyces is considered' (Montrocher et al., 1998). In contrast, the Saccharomyces sensu lato group is more diverse consisting of S. dairensis, S. castelli, S. exiguus, S. servazzii and S. unisporus. This group is genetically heterogeneous both in terms of chromosome number (7-16) and genome size. Accordingly and somewhat inevitably, these yeasts can be further subdivided into four groups (Petersen et al., 1999). The third group consists of one species, S. kluyveri, which increasingly is being consolidated within the Saccharomyces sensu lato group (Marinoni et al., 1999; Petersen et al., 1999). Whatever, S. kluyveri is the most divergent member of the genus with only 5 or 7 chromosomes (Vaughan-Martini & Martini, 1998; Petersen et al., 1999) and, within the Saccharomyces, a unique fatty acid profile (Augustyn et al., 1991).

Table 4.8 Split of the Saccharomyces sensu stricto - commonality and differences between the species.

Habitat

Genome size

Maximum growth temperature (°C)

Fructose transport

Melibiose utilisation rDNA spacer sequences

S. cerevisiae

Domesticated -brewing (ale), baking, enology

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