BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

Part IV INFORMATION

CHAPTER 28. THE REGULATION OF GENE EXPRESSION IN THE PHENOTYPE

Summary

Cells regulate The amount of synthesized Proteins in various ways. In E. coli, Gene Expression is regulated primarily at the level of METABOLISM/31.html">Transcription rather than Translation. Many genes are organized into operons, which are coordinated units of genetic expression. An Operon consists of regulatory regions (an operator and a promoter) and several structural genes. Outside the operon lies a regulatory gene that encodes a protein interacting with the operator site. The base sequence of the lactose operator is symmetrical. In general, Symmetry plays a fundamental role in the recognition of specific sites on DNA by proteins. The lac operon is induced by ß-galactosides, such as allolactose and isopropyl thiogalactoside. The binding of the inducer to the lac repressor causes its displacement from the operator. RNA polymerase can then pass through the operator and transcribe the lac operon. The Tryptophan operon is repressed by tryptophan, which binds to a specific repressor, thereby enabling it to bind to the operator. As a result, the transcription of genes encoding Enzymes for tryptophan Biosynthesis is turned off.

In addition, some amino acid biosynthetic operons, including the trp operon, are under the control of attenuators. If the end product of biosynthesis—The amino acid—is abundant, transcription is terminated at the attenuator region. Attenuation requires the translation of a leader mRNA. The regulation of a wide range of genes is mediated by cyclic AMP, which binds to a specific protein (known as CAP). This complex interacts with the promoter regions of several inducible operons and stimulates Transcription initiation. The concentration of cyclic AMP increases only when glucose is deficient. Thus, glucose indirectly represses the synthesis of various catabolic enzymes.

Bacteriophage can replicate and destroy host cells (the lytic pathway); alternatively, its DNA can integrate into the host Cell chromosome (the lysogenic pathway). Lytic development involves the sequential transcription of three groups of genes. At the immediate-early stage, protein N is produced, which activates the transcription of early genes. This, in turn, synthesizes protein Q, an activator of late-stage transcription. The lysogenic state is maintained by the repressor, which is encoded by the cI gene. The repressor binds to the OR and O_L operators and prevents the transcription of immediate-early genes. The presence of Multiple binding sites in these operators enables the repressors to regulate their own synthesis.