Chapter 18- Control of Gene Expression in Bacteria 

18.1 An Overview of Gene Regulation and Information Flow

• The flow of information from DNA to active protein suggest three possible mechanisms:
  • Transcriptional control: The cell could make mRNAs only for proteins it needs.
  • Translational control: The cell could prevent the mRNAs for unneeded proteins from being translated.
  • Post-translational control: After translation, many proteins have to be activated by chemical modification, such as the addition of a phosphate group, in order to function.
• An inducer is a small molecule that triggers transcription of a specific gene. 

18.2 Negative and Positive Control of Transcription

• Negative control occurs when a regulatory protein called a repressor binds to DNA and shuts down transcription 
• Positive control occurs when a regulatory protein called an activator binds to DNA and triggers transcription
• Cells that are abnormal because they produce a product at all times instead of regulating expression of the product are called constitutive mutants. 
• Jacob and Monod coined the term operon to describe a set of coordinately regulated bacterial genes that are transcribed together into one polycistronic mRNA. 
• The group of genes involved in lactose metabolism was termed the lαc operon.
• The repressor binds to a DNA sequence in the lαc operon called the operator.
• In allosteric regulation, a small molecule binds to a protein and causes it to change its shape and activity.
• A protein known as the catabolite activator protein (CAP) has long been viewed as exerting positive control of many operons in E. coli, including the lαc operon. 
• Negative feedback control is a form of control in which the final product of a pathway inhibits the production of the product.
• The only difference is that for the trp operon the small molecule regulator is called a co-repressor, because it works with the repressor to make it active. 

18.3 Global Gene Regulation

• Global gene regulation is the coordinated regulation of many genes. 
• Regulon is a set of separate genes and operons at contain the same regulatory sequences and are controlled by a single type of regulatory protein
• ==A key part of the regulatory logic of the SOS system is that it is under the control of one of its own genes.==

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