Yeast Research: A Historical Overview

We know that an analogous decomposition of salicin is produced by emulsin; neither in the one case nor in the other, is it possible to detect a physiological act. But between 1855 and 1875 Pasteur established unequivocally (i) the role of yeast in alcoholic fermentation, (ii) fermentation as a physiological phenomenon, and (iii) differences between the aerobic and anaerobic utilization of sugar by yeasts. The first part of Pasteur's paper deals with the changes in sugar which are brought about by alcoholic fermentation. The second part considers especially the "ferment", its nature, and the transformations it undergo. Pasteur's work on beer and wine yeasts gives some account of different yeasts, although he was never much interested in taxonomy. Indeed, his work Études sur la Bière described some elegant experiments on yeasts associated with wine grapes, probably carried out in the autumn of 1872. In 1862 Pasteur had discussed the sources of wine yeasts, describing how yeasts could be found in different fruit juices of high acidity, although if the juices were less acidic, bacteria would grow too. By 1880, alcoholic fermentation as a sign of the physiological activity of yeasts was not quite yet scientific orthodoxy. Up to that time, the finding of independent enzymic activity, separated from that of living cells, impeded understanding of the role of enzymes in cellular activity.

This chapter gives a coherent account of the gradual and uneven emergence of understanding of yeast cytology. Indeed, although the vacuole is usually the largest organelle in a yeast cell, Guilliermond was absolutely right: for many years, the identities of the yeast vacuole and nucleus were confused by a number of authors. The vacuole, it is now known, is bounded by a single membrane, the tonoplast, and acts as a lysosome, particularly for nonspecific intracellular proteolysis. The ascospores (the products of meiotic cell division) had long been known to exist in certain yeasts and other fungi, but their sexual significance was not understood. In 1891, Hansen published a paper on ascospore germination in S. cerevisiae, Saccharomycodes ludwigii, and Pichia anomala. Certain specialized yeast cells, ballistoconidia and chlamydospores, were clearly described at the end of the 19th century, but studies on yeast dimorphism have been done mostly in the 20th century. Since 1950, the existence and characteristics of most organelles of yeasts, such as mitochondria, Golgi apparatus, and endoplasmic reticulum, have been established largely by the use of phase-contrast and electron microscopy, as well by advances in biochemistry, genetics, and molecular biology.

This chapter on fermentation pathway during the period 1900 to 1950, records how the metabolic pathway of alcoholic fermentation was gradually revealed during the first half of the 20th century and provides chronological summary of the metabolic pathway. During the first half of the 20th century, the role of phosphates in glycolysis was studied extensively and the course and nature of alcoholic fermentation by yeasts and of lactic acid production by muscles were thus uncovered. This research provided the key to understand other metabolic processes, including the energy-transforming machinery of living cells. The first account of the formation of ATP, produced at the expense of energy derived from glycolysis, was published in 1934 when Jacob Karol Parnas, Pawel Ostern, and Thaddeus Mann reported the transfer of phosphate from phosphoglycerate to the adenylic acid system in muscle and Karl Lohmann and Otto Fritz Meyerhof discovered phosphoenolpyruvate. Although this chapter names only some of the many research workers and lists merely a few of the relevant publications, it summarizes the enormous amount of research on glycolysis carried out in the first half of the 20th century. The intricate and labyrinthine story of elucidating the fermentation pathway is complicated by the involvement of two systems, alcoholic fermentation by yeasts and lactic acid fermentation by muscles.

Most of the work which explained the general phenomenon of enzymic adaptation was done by studying the microbial utilization of lactose and d-galactose, both of which have long been relatively easy to obtain and purify. This chapter concentrates on the utilization of these two sugars. Three different lines of research have provided the basis for understanding adaptations by yeasts and other microbes. The first was the work on galactose utilization by yeasts, published by Frédéric Dienert in 1899 and 1900. Second, the remarkable adaptive pathway of d-galactose catabolism by yeasts was worked out by Luis Leloir and his colleagues between 1948 and 1952. Finally, in the 1950s and 1960s, Jacques Monod, François Jacob, André Lwoff , and their confrères explained microbial adaptations largely in terms of induction and repression of enzyme synthesis, regulated by a complex of genes. By the late 1940s, the fermentation of galactose by yeasts had become the most thoroughly studied system of enzymic adaptation. Georges Cohen and Monod argued that the entry of organic substrates into microbial cells is mediated by more or less selective permeation systems, which they proceeded to characterize. During the 1950s, Monod and his colleagues isolated many mutants from Escherichia coli. In 1956, their seminal findings were published, which led them to the concept of "permeases". The genetic regulatory mechanism of Saccharomyces cerevisiae, acting on the GAL genes which encode the enzymes of galactose utilization, has been the most intensively studied and has become the best-understood genetical regulatory mechanism in any eukaryote.

This chapter on metabolite transport by facilitated diffusion describes some of the history of studying how molecules move across cell membranes, particularly yeast membranes. In discussing how substances both enter and leave yeast cells, it is necessary to consider the structures through which the substances must pass, namely, the cell wall and the plasma membrane, as well as intracellular membranes. Steveninck and Rothstein suggested that while uptake involves sugar phosphorylation, when glycolysis is prevented by iodoacetate, sugar uptake occurs by facilitated diffusion. The results of pulse-labeling experiments indicated that uptake of galactose by baker’s yeast was adaptive and probably involved entry of the free sugar and its accumulation to diffusion equilibrium. The following three independent findings were consistent with this conclusion. First, the high-affinity mode of galactose uptake was found to depend on the presence of galactokinase. Second, from pulse-labeling studies of glucose uptake by a wild-type yeast, via a carrier of low Km, Kotyk published convincing evidence that free sugar in the intracellular pool was labeled first. Finally, the route with a high Km, in a kinaseless yeast strain, conformed to the established pattern of a facilitated diffusion pathway. The results suggest that high-affinity glucose transport is not necessarily dependent on the presence of glucose-phosphorylating enzymes. Apparent low-affinity uptake kinetics can arise as a consequence of an insufficient rate of removal of intracellular free glucose by phosphorylation.

This chapter provides an account of some of the early works on yeasts which laid the foundations for several major developments in genetics as a whole. Much of the pioneer and totally academic work on yeast genetics was done at the Carlsberg Laboratory in Copenhagen between 1935 and 1955, largely by the Danish geneticist Øjvind Winge, who can reasonably be regarded as the founder of yeast genetics. In 1939, Winge and Laustsen confirmed Guilliermond’s earlier suggestion that Saccharomycodes ludwigii is heterothallic. Working with Neurosporacrassa, in 1933, Lindegren had suggested that the frequency of crossing-over between a gene and its centromere was a measure of their distance apart on the linkage map. It should be mentioned here that in 1985, electrophoretic karyotyping showed that Saccharomyces cerevisiae has a haploid number of 16 chromosomes, and this was confirmed later. In the absence of sexual reproduction, genetic mapping begun in the early 1980s, by fusing spheroplasts, so that recombination analyses became practicable. Work on the cell cycle in S. cerevisiae and Schiz. pombe has enormously increased the understanding of how cell growth is controlled and, hence, has provided information which will assist in developing the medical control of human cancers. The genetic tractability of S. cerevisiae and its nonpathogenic character have made it attractive for elucidating cellular biochemistry and facilitating the molecular analysis of genes which cause diseases. It is used for testing antifungal products and other new drugs.

As well as isolating new species, taxonomists have, almost uniquely undertaken the comparative study and description of many different kinds of yeast, and this is probably one of their most valuable contributions to yeast biology. This chapter considers some aspects of the history of all these kinds of activity. Ascospore-forming yeasts, such as Saccharomyces species, were isolated notably from industrial fermentations, whereas many non-ascospore-producing yeasts were found in clinical practice-for example, Candida albicans and Malassezia furfur, often the putative causes of mycotic diseases. In 1924, Endomyces fibuliger was transferred to Saccharomycopsis. However, the genus Saccharomycopsis generally lapsed into desuetude until Kreger-van Rij restored it in all its glory in 1984. Writing from the Faculté de Médecine of Paris in 1932, Maurice Langeron and Rodolfo Talice published a paper on classifying those fungi which characteristically formed both filaments and yeast-like cells. This paper was largely a report of a microscopical study of the different categories of cell produced by each kind of yeast: blastoconidia, chlamydospores, the mode of budding, and the greatly varied appearance of filamentous growths. The chapter describes the inception of some genera which were thought to be asexual, namely, Brettanomyces, Sporobolomyces, Bullera, Rhodotorula, Kloeckera, Trigonopsis, and Schizoblastosporion. The sensible naming of yeasts is vital for all who work with them, in research, in commerce, and in medicine.