Summary: When and where do cells have control over which genes are expressed and which proteins are active? Proteins are activated through processing and signals can enlist cellular machinery to destroy proteins Proteins determine when and how often RNA polymerase can bind Modifications to pre-mRNA affect the life and use of RNA in the cytoplasm

Newer Ideas to Look For The study of epigenetics investigates ways that whole sections of chromosomes are activated and deactivated.

Looking at Individual Genes The simplest and most well understood model of the regulation of individual genes is found in prokaryotes. We will study two examples of gene control in E. coli ; a bacteria found in the human digestive tract

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The Operon Coding Region: Gene(s) Control Region Operon

The Lac Operon in E. coli Genes Control Region Produce the enzymes necessary for digesting lactose Respond to the presence or absence of lactose and glucose

The Lac Operon in E. coli lacZlacYlacA operator promoter activator ON OFF Each lac gene codes for an enzyme only needed when lactose is present FAST SLOW Binding site for RNAP (CAP binding site)

The Lac Operon in E. coli

With no proteins bound to either the operator or activator regions, RNAP is free to bind to the promoter and transcribe the genes lacZlacYlacA OPA pre-mRNA

The Lac Operon in E. coli If there is no lactose present, this would be a waste of valuable resources like ATP and amino acids lacZlacYlacA OPA pre-mRNA

The Lac Operon in E. coli To prevent transcription, the genes can be “turned off” when a repressor protein is bound to the operator sequence lacZlacYlacA OPA repressor GET OUT OF THE WAY!

The Lac Operon in E. coli When lactose is present, it will bind to the repressor, change its 3D shape and remove it from the operator. lacZlacYlacA OPA

The Lac Operon in E. coli When lactose is present, it will bind to the repressor, change its 3D shape and remove it from the operator. lacZlacYlacA OPA

The Lac Operon in E. coli RNAP is now free to bind the promoter and transcribe the sequence lacZlacYlacA OPA

The Lac Operon in E. coli Since the presence of lactose removes the repressor and “turns on” the genes, lactose is called an inducer. lacZlacYlacA OPA repressor inducer

What it “really” looks like

The Lac Operon in E. coli Operon is OFF lacZlacYlacA OPA

The Lac Operon in E. coli Operon is ON lacZlacYlacA OPA

The Lac Operon in E. coli Glucose is the preferred energy source for cells glucose lactose Cell OM NOM NOM NOM NOM

The Lac Operon in E. coli If glucose levels are LOW, lactose metabolism is prioritized If glucose levels are HIGH, lactose metabolism is of lower priority glucose LOW HIGH How can glucose levels be detected in the nucleus?

cAMP and CAP cAMP binds to the activator protein “CAP” (Catabolite activator protein) Together they will bind to DNA ahead of the promoter to help RNAP bind

cAMP and CAP When glucose levels are low, cAMP activated CAP proteins will stimulate the transcription of over 100 genes including the lac operon

The Lac Operon in E. coli Without the cAMP-CAP complex bound to the activator, RNAP is less likely to bind and produces fewer transcripts in a given time lacZlacYlacA OPA “SLOW”

The Lac Operon in E. coli With the cAMP-CAP complex bound to the activator, RNAP binds more readily, speeding up the production of RNA transcripts lacZlacYlacA OPA “FAST”

3D model of cAMP-CAP complex bound to DNA /Catabolite_gene_activator_protein

Summary  glucose lactose ON / OFF FAST / SLOW  lacZlacYlacA OPA   lacZlacYlacA OPA lacZlacYlacA OPA lacZlacYlacA OPA OFF ON FAST SLOW

The trp Operon in E. coli If ingested levels of the amino acid tryptophan are low, e. coli is able to synthesize it on its own. When tryptophan levels are low, the try operon is turned on to make the enzymes necessary to synthesize more.

The trp Operon in E. coli Again, the cells should not waste resources making more tryptophan if it is already present

The trp Operon in E. coli No tryptophan = operon on Enzymes are synthesized to make trp trpE OP trpDtrpCtrpBtrpA

The trp Operon in E. coli Tryptophan present = operon off Trp is a co-repressor (needed for repressor to function) trpE OP trpDtrpCtrpBtrpA repressor co-repressor

What it “really” looks like

Negative gene regulation Repressors are used to turn off genes lacZlacYlacA OPA repressor inducer repressor co-repressor trpE OP trpDtrpCtrpBtrpA

Negative Gene Regulation The lac Operon

Positive Gene Regulation The lac Operon

On your own, summarize the post transcriptional and post translational control cells have over their genes using your textbook

Co-Repressor