BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

Part IV INFORMATION

CHAPTER 28. THE REGULATION OF GENE EXPRESSION IN THE PHENOTYPE

We have already seen that The activity of many Proteins is regulated by various mechanisms, such as proteolytic activation, allosteric interactions, and covalent modification. This chapter examines the Regulation of the rate of Protein Synthesis, which also plays a fundamentally important role in the overall picture of cellular METABOLISM. In Bacteria, Gene activity is regulated primarily at the level of Transcription rather than Translation. We will focus on the lactose and Tryptophan operons of E. coli and on the regulatory aspects of the bacteriophage development cycle, as the MOLECULAR MECHANISMS OF regulation in these systems are well understood. Furthermore, intensive research into these systems has made it possible to formulate several General Principles of the Introduction/30.html">Regulation of Gene Expression in the phenotype among prokaryotes and Viruses. Gene Expression IN eukaryotes is regulated differently, as will become evident in the next chapter.

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28.1. β-Galactosidase is an Inducible Enzyme

E. coli can utilize lactose as its sole carbon source.

The key enzyme in the metabolism of this sugar is β-galactosidase, which hydrolyzes lactose into galactose and glucose (Fig. 28.1). When grown on lactose, an E. coli Cell contains several thousand molecules of β-galactosidase. Conversely, when E. coli is grown on other carbon sources, such as glucose or glycerol, the number of β-galactosidase molecules per cell drops below ten. Lactose induces a significant increase in The amount of β-galactosidase in the E. coli cell, and it triggers the synthesis of new enzyme molecules rather than activating a proenzyme (Fig. 28.2). Consequently, β-galactosidase is an inducible enzyme. Simultaneously and coordinately with β-galactosidase, two other proteins are synthesized: galactoside permease and thiogalactoside transacetylase. The permease is required for The transport of lactose across the bacterial cell membrane, whereas the transacetylase is not essential for lactose metabolism. The Physiological Role of transacetylase has not yet been established; in vitro, it catalyzes The transfer of an acetyl group from acetyl-CoA to the hydroxyl group at C-6 of thiogalactosides.

Fig. 28.1. β-Galactosidase hydrolyzes lactose

Fig. 28.2. The increase in the amount of β-galactosidase parallels the increase in cell number in a growing culture of E. coli. The slope of this plot indicates that β-galactosidase accounts for 6.6% of the total synthesized protein

The physiological inducer of β-galactosidase is allolactose, which is formed from lactose via a transglycosylation reaction. The synthesis of allolactose is catalyzed by the few molecules of β-galactosidase already present in The Cell prior to induction. Studies on The Nature of Inducers have shown that some β-galactosides act as inducers without serving as substrates for β-galactosidase, whereas Other Compounds behave as substrates without being inducers. For example, isopropylthiogalactoside (IPTG) is a non-metabolizable inducer (also referred to as a gratuitous inducer).