Genome Regulation: Gene Expression

Regulation of Gene Expression

• Gene expression is altered in response to changing environments in both prokaryotes and eukaryotes.
• In multicellular eukaryotes, gene expression regulates development and is responsible for differences in cell types.

Differences between Gene Expression in Prokaryotes and Eukaryotes

• Prokaryote gene expression is typically regulated by an operon, a collection of controlling sites adjacent to polycistronic protein-coding sequences.
• Eukaryotic genes are also regulated in units of protein-coding sequences and adjacent controlling sites, but operons are not known to occur.
• Eukaryotic gene regulation is more complex due to the presence of a nucleus.

Types of Eukaryotic Gene Regulation

• Short-term regulation: genes are quickly turned on or off in response to the environment and demands of the cell.
• Long-term regulation: genes are involved in development and differentiation.

Operon Model

• A cluster of functionally related genes can be under coordinated control by a single "on-off switch".
• The regulatory "switch" is a segment of DNA called an operator, usually positioned within the promoter.
• An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control.

Regulation of Operon

• The operon can be switched off by a protein repressor.
• The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase.
• The repressor is the product of a separate regulatory gene.
• The repressor can be in an active or inactive form, depending on the presence of other molecules.

trp Operon

• The trp operon is responsible for the synthesis of the amino acid tryptophan.
• The operon is switched off when tryptophan is present, and switched on when tryptophan is absent.

Repressible and Inducible Operons

• A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription.
• The trp operon is a repressible operon.
• An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription.
• The lac operon is an inducible operon and contains genes that code for enzymes used in the hydrolysis and metabolism of lactose.

lac Operon

• The lac operon is usually off, but is turned on when lactose is present.
• The lac repressor is active and switches the lac operon off when lactose is absent.
• Allolactose, an inducer, inactivates the repressor to turn the lac operon on.
• Inducible enzymes usually function in catabolic pathways; their synthesis is induced by a chemical signal.