Gene Regulation + Operons

promoter

sequence of genes that initiates the transcription of mRNA

• helps RNA locate the right spot to start transcription

pGLO

• A plasmid (circular DNA) used to genetically modify bacteria, engineered to incoporate aspects of the arabinose operon.

• contains: the GFP gene, a bla gene (provides ampicillin resistance), parts of the arabinose operon (PBAD and araC gene) that regulates GFP expression based on the presence of arabinose

four major parts of operons

structural genes, promoter genes, the operator, and the regulatory gene

operator

regulatory DNA sequence where repressors bind to control gene expression.

operon

A cluster of genes transcribed as a single mRNA, regulated together.

repressor

binds to the operator to block RNA polymerase (mRNA can’t be made, “off” switch")

regulatory gene

codes for repressor → produces them slowly

positive gene regulation

a transcription factor/activator protein binds at the promoter → helps RNA polymerase to initiate transcription more effectively

How does negative gene regulation relate to operons? (Negative feedback)

repressor protein that binds to the operator to prevent transcription.

• When the repressor is active, it blocks RNA polymerase from transcribing the operon’s genes.

• A signal molecule (corepressor or inducer) can influence the repressor’s activity, either activating or inactivating it.

• helps maintain homeostasis by ensuring genes are only expressed when needed

• Example: In some operons, when a product accumulates, it activates the repressor, turning off transcription (negative feedback loop).

structural genes

genes within an operon that code for proteins

corepressor

• molecule that binds to a repressor, making it active so that it can bind to the operator region of a repressible operon, thereby blocking RNA polymerase and preventing transcription of the structural genes (e.g., helps turn an operon off).

active repressor

• binds to the operator region of a gene, preventing gene expression by blocking RNA polymerase from transcribing the gene

inactive repressor

• regulatory protein that is NOT bound to its target DNA sequence

  • allows gene expression to proceed as normal

functions of gene regulation

• cell specialization

• developmental changes

• adaption environment

• prevents overproduction of certain proteins

how are genes regulated

• switching transcription on/off

• happens at promoter or operator

• related genes grouped together and use the same promoter + operator

describe the arabinose operon in e.coli

• contains genes coding for enzymes used to breakdown arabinose

• Controlled by the araC regulatory protein.

• In the pGLO plasmid, the GFP gene replaces the arabinose metabolism genes, so GFP is only expressed when arabinose is present.

difference between +pGLO and -pGLO

+pGLO (with plasmid):

• Bacteria contain the pGLO plasmid.

• If grown on LB + ampicillin, only transformed bacteria survive.

• If grown on LB + ampicillin + arabinose, bacteria glow green (GFP expression is induced).

-pGLO (no plasmid):

• Bacteria do not contain the pGLO plasmid.

• Cannot survive on LB + ampicillin (no resistance gene).

• No GFP expression (even if arabinose is present).

Why does heat shock/cold treatment work in getting a bacteria to uptake a plasmid?

rapid change in temperature temporarily opens pores in the membrane, allowing the pGLO plasmid to enter the cell.

inducer

molecule that inactivates a repressor, allowing transcription to occur. (like arabinose for the arabinose operon)