The Lac Operon and Gene Regulation

Vocabulary flashcards covering the structure, function, mutants, and experimental verification of the E. coli Lac operon as described by Jacob, Monod, Gilbert, and M\u00fcller-Hill.

Lac operon

The first model of gene regulation discovered in E. coli, focusing on the utilization of lactose.

Transcription

The core biological process involving the synthesis of mRNA from a DNA template.

Translation

The core biological process where mRNA is translated into a specific protein.

Housekeeping genes

Genes that are expressed constitutively (always on), such as structural proteins and enzymes for the Krebs cycle or glycolysis.

Regulated genes

Genes that are switched on and off as required; their expression is described as spatial and temporal (where and when).

Developmental genes

A specialized version of regulated genes switched on at particular stages of the cell cycle, critical for cell division and sporulation in bacteria.

Operon

A structure unique to prokaryotes consisting of a group of genes transcribed as a single mRNA transcript that is then translated into multiple proteins.

lacZ (\beta-galactosidase)

The enzyme responsible for the cleavage of lactose into galactose and glucose.

lacY (Lactose permease)

A permease required for the high affinity uptake of lactose into the cell by forming channels in the membrane.

lacA (Galactoside acetyl transferase / GAT)

An enzyme that assists in cellular detoxification by acetylating galactosyl, an intermediate of galactose catabolism.

Inducer

A molecule, such as lactose or the chemical analogue IPTG, that triggers the expression of specific proteins.

IPTG (Isopropyl \beta-D-1-thiogalactopyranoside)

A synthetic lactose analogue that serves as a better inducer than lactose because it is not cleaved by \beta-galactosidase.

ONPG (2-Nitrophenyl \beta-D-galactopyranoside)

A substrate used in assays for \beta-galactosidase activity that produces a yellow o-nitrophenol product when cleaved.

Rifampicin

An antibiotic used to prove regulation occurs at the level of transcription because it inhibits the synthesis of mRNA.

Chloramphenicol

An antibiotic that inhibits translation and was used in experiments to determine the mechanism of induction.

Positive control mechanism

A hypothesis where an inducer like lactose identifies a control site (promoter) to stimulate the activity of RNA polymerase.

Repressor protein (LacI)

A protein that binds DNA at the operator site to stop RNA polymerase from transcribing the lac operon.

NULL mutants

Mutations that lead to defective proteins that no longer work correctly, resulting in an absence of function.

LacI- mutant

A constitutive mutant where the repressor protein is non-functional and cannot bind DNA, leading to the operon being always ON.

IS mutant

A super-repressed mutant where the repressor has a mutation in the lactose-binding domain, meaning it never dissociates from DNA.

Operator site

A specific DNA sequence in the lac promoter region recognized and bound by the lac repressor protein.

OC mutant

An operator constitutive mutant where the DNA sequence is changed so the repressor cannot recognize or bind it, resulting in the operon being always ON.

Equilibrium dialysis

An assay developed by Gilbert and M\u00fcller-Hill to isolate the repressor protein using a radioactive lactose analogue ($C^{14}$ IPTG).

DNA footprinting

A biochemical method for determining protein-binding regions by using DNAse I to digest DNA not protected by a bound protein.

Catabolite repression

The effect where glucose overrides the induction of the lac operon, ensuring the most energy-efficient carbon source is used first.

CAP (Catabolite Activator Protein)

A protein that binds cAMP to stabilize RNA polymerase binding at the promoter, enabling strong transcription.

cAMP (Cyclic AMP)

A signaling molecule whose concentration drops when glucose is present, leading to the dissociation of CAP and weak transcription of the lac operon.