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Chapter IX: Glucose Effects: Inducer Exclusion and Repression

Boris Magasanik


The inhibitory effect of glucose on the formation of β-galactosidase by E. coli was recognized at an early date by Monod (1947). He observed diauxic growth of the organism in a medium containing a mixture of glucose and lactose as sources of carbon. The phenomenon is illustrated in Fig. 1, from a later paper by Epstein, Naono, and Gros (1966). It can be seen that the synthesis of the enzyme is almost completely inhibited during the first phase of rapid growth on glucose. The differential rate of enzyme synthesis is highest during the period of very slow growth that follows the exhaustion of glucose. Enzyme synthesis continues at a some-what lower differential rate when rapid growth resumes on lactose.

β-galactosidase is not the only enzyme whose rate of synthesis is reduced by the presence of glucose in the medium. It has been found that glucose interferes generally with the synthesis of catabolic enzymes other than those involved in the degradation of glucose by enteric bacteria, bacilli, yeasts, and other microorganisms (Magasanik, 1961).

I shall attempt to show in this chapter that glucose interferes with the synthesis of β-galactosidase in inducible strains of E. coli in three ways. It excludes the inducer from cells that do not contain a high level of the lac y gene controlled permease; it represses β-galactosidase strongly, but transiently, when added to cells growing on another source of carbon (transient repression); it represses the enzyme weakly but permanently during balanced growth (catabolite repression). Both repressive...

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