14: Gene Regulation in Bacteria

Gene regulation

Modifying expression of genes in different conditions

Constitutive genes

Genes that are unregulated and always expressed

What types of genes are constitutive genes

Essential genes

Factors that influence gene regulation changes

• Metabolism

• Environmental stress

• Cellular division

Gene regulation at the transcriptional level

Regulate the number of copies of RNA made

Gene regulation at the translational level

• Repress translation

• Keep translation from starting

• Degrade the mRNA

Gene regulation at the post-translational level

Protein activation contingent on modifications

Most common way to regulate genes

Transcriptional regulation

Proteins that regulate transcription

Activators and repressors

Repressor

Binds to DNA at promoter region to negatively regulate transcription

Activator

Binds to DNA at promoter region to positively regulate transcription

Inducer

Molecule that upregulates transcription by activating activators or inactivating repressors

Corepressor

Binds to repressors and activates them, downregulating transcription

Inhibitor

Binds to activators and deactivates them, downregulating transcription

Operon

A regulatory unit with multiple genes behind one promoter (polycistronic)

Regulatory DNA components in an operon

• Promoter

• Operator

• CAP site

What does the promoter bind

DNA Pol

What does the operator bind

Repressor and other regulating molecules

What does the CAP site bind

Catabolic activator protein

What is the catabolite activator protein complexed to in E. coli

cAMP

Genes in the lac operon

Lac z, y, and a

Lac z

Encodes β-galactosidase

Lac y

Encodes lactose permease, the transmembrane protein that brings lactose into the cell

Lac a

Encodes galactoside transacetylase, which modifies lactose in the cell

Inhibitor gene for the lac operon

Lac I

How does Lac I inhibit the lac operon

Codes for the lac repressor

How is the lac operon activated

It is constitutionally repressed by Lac I. When lactose or allolactose is present, it binds to the repressor, acting as an inducer. The repressor releases, allowing for RNA pol to bind to the Lac operon promoter, resulting in the expression of the lac operon

How does E. coli use lactose and glucose

It preferentially uses glucose

How does E. coli use catabolite suppression in the digestion of glucose and lactose

The presence of glucose inhibits adenylyl cyclase, which decreases the concentration of cAMP, so there is no active CAP-cAMP complex for the promoter, which prevents expression of the lac operon

Lac operon transcription when glucose is low and lactose/allolactose is high

High transcription

Lac operon transcription when glucose is low and lactose/allolactose is low

No transcription

Lac operon transcription when glucose is high and lactose/allolactose is high

Low transcription

Lac operon transcription when glucose is low and lactose/allolactose is low

Low transcription

trp operon function

Tryptophan biosynthesis

trp operon regulatory genes

• trp R: encodes trp repressor protein

• trp L: Encodes short leader peptides that help attenuate transcription

How is the trp operon transcription regulated

The presence of tryptophan down regulates operon transcription

How does the attenuation sequence contribute to trp operon regulation

The attenuation sequence helps regulate by interfering with translational activity, which can be coupled to transcription. High levels of tryptophan cause the sequence to fold in a way that kicks off the RNA pol and prevent over expression (through translation) of the operon.

Major way to regulate translation

Translational regulatory proteins

Ways that translational regulatory proteins inhibit translation

• Binds to start codon (or shine-dalgarno sequence) and sterically hinders ribosomal binding

• Binds outside the start codon or shine-dalgarno sequence, reinforcing secondary structure that inhibits translation

Anti-sense RNA

RNA that is the reverse complement of mRNA, forming dsRNA, which is quickly degraded by the cell

Gene that encodes for anti-sense RNA

Mic F

Common posttranslational modification

• Phosphorylation

• Acetylation

• Methylation

Enzyme binding sites needed for allosteric regulation

Catalytic site and regulatory site