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# Regulation of Gene Expression * **Gene expression** is the process by which DNA directs protein synthesis. * Gene expression includes transcription and translation. * Gene expression is regulated to ensure proteins are produced only when needed. ## I. Bacteria: Regulation of Gene...

# Regulation of Gene Expression * **Gene expression** is the process by which DNA directs protein synthesis. * Gene expression includes transcription and translation. * Gene expression is regulated to ensure proteins are produced only when needed. ## I. Bacteria: Regulation of Gene Expression ### A. Overview * **Operon**: entire stretch of DNA that includes the operator, the promoter, and the genes they control * Can be switched off by a **repressor** * **Repressor**: protein that binds to the operator and blocks attachment of RNA polymerase to the promoter * Prevents transcription * The repressor is a product of a separate **regulatory gene** * **Corepressor**: a small molecule that binds to a bacterial repressor protein and changes the protein's shape, allowing it to bind to the operator and block transcription ### B. Types of Operons * **Repressible operon**: transcription is usually on but can be inhibited (repressed) when a specific small molecule binds allosterically to a regulatory protein * Example: *trp* operon (for synthesis of tryptophan) * Tryptophan absent: repressor inactive; *trp* operon on * Tryptophan present: tryptophan binds to the *trp* repressor protein, which turns the operon off. * **Inducible operon**: transcription is usually off but can be stimulated (induced) when a specific small molecule interacts with a regulatory protein. * Example: *lac* operon (for breakdown of lactose) * Lactose absent: repressor active; *lac* operon off * Lactose present: lactose binds to the *lac* repressor protein, which turns the operon on. * **Inducer**: Inactivates the repressor ### C. Positive Gene Regulation * **Positive gene regulation**: regulatory protein interacts directly with the genome to switch transcription on. * Example: *lac* operon * When glucose is scarce, *E. coli* can use lactose * **CAP** (catabolite activator protein): activator protein that binds to DNA * cAMP (cyclic AMP): accumulates when glucose is scarce, activates CAP * Activated CAP binds to promoter region * Increases RNA polymerase affinity for promoter * Increases transcription * When glucose is plentiful, CAP detaches from the operon, and transcription proceeds at a low rate. ## II. Eukaryotes: Regulation of Gene Expression ### A. Overview * Gene expression can be regulated at any stage * **Chromatin modification** * **Transcription** * **RNA processing** * **Translation** * **Protein processing and degradation** ### B. Chromatin Modification * **Chromatin**: DNA and protein * **Heterochromatin**: highly condensed chromatin * Genes usually not expressed * **Euchromatin**: loosely packed chromatin * Genes usually expressed * **Histone acetylation**: acetyl groups attach to histones * Loosens chromatin structure * Promotes initiation of transcription * **DNA methylation**: addition of methyl groups to DNA * Condenses chromatin * Reduces transcription ### C. Transcription * **Control elements**: segments of noncoding DNA that serve as binding sites for transcription factors * **General transcription factors**: essential for the transcription of all protein-coding genes * **Specific transcription factors**: include activators and repressors * **Activators**: bind to control elements and stimulate transcription * **Repressors**: bind to control elements and block transcription ### D. RNA Processing * **RNA processing**: modification of RNA transcripts * **Alternative RNA splicing**: different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns. ### E. Translation * **Translation**: synthesis of polypeptide * Regulation of translation can occur via * Initiation factors * Regulatory proteins that bind to mRNA * miRNAs (microRNAs) and siRNAs (small interfering RNAs) * Can bind to mRNA and degrade it or block its translation ### F. Protein Processing and Degradation * **Protein processing**: modification of polypeptide * Regulation can occur via * Protein folding * Chemical modifications * Transport to cellular destination * **Proteasomes**: giant protein complexes that bind protein molecules and degrade them * **Ubiquitin**: small protein that marks proteins for degradation