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Chapter 8 : Enzymic Adaptation and Regulation, 1900 to 1960
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.