Biol 3130 Lecture 10: The Lac Operon

Flashcards covering key vocabulary and concepts from the lecture on the control of gene expression in prokaryotes, specifically focusing on the Lac Operon.

Lac Operon

A genetic regulatory system in E. coli that controls the expression of genes involved in lactose metabolism, consisting of structural genes and regulatory elements.

Negative Inducible Lac Operon

A type of lac operon regulation where a repressor protein normally blocks transcription, but in the presence of an inducer (lactose/allolactose), the repressor is removed, and transcription is turned 'ON'.

Positive Regulation of Lac Operon

Regulation of the lac operon that involves Catabolite Activator Protein (CAP) enhancing the transcription rate when glucose levels are low.

Catabolite Activator Protein (CAP)

A protein that binds upstream of the lacP promoter in its active form (when bound to cAMP) to enhance the transcription rate of the lac operon.

Jacob and Monod

Scientists awarded the Nobel Prize for their seminal work using elegant mutant analyses to identify and characterize the different elements and functions of the lac operon.

Regulator Gene (lacI)

The gene that encodes the repressor protein (LacI) for the lac operon, which binds to the operator.

Promoter (lacP)

A regulatory DNA sequence in the lac operon where RNA Polymerase binds to initiate transcription.

Operator (lacO)

A regulatory DNA sequence in the lac operon where the repressor protein binds to block RNA Polymerase activity.

Structural Genes (lacZ, lacY, lacA)

Genes within the lac operon that encode enzymes for lactose metabolism: β-galactosidase, permease, and transacetylase.

cAMP (cyclic AMP)

An important second messenger molecule that, when levels are high due to low glucose, binds to CAP, forming the cAMP-CAP complex to activate lac operon transcription.

Haploid

A cell or organism having a single copy of the genome per cell, typical for most prokaryotes.

Diploid

A cell or organism having two copies of the genome per cell, typical for somatic cells of eukaryotes.

Partial Diploid

An E. coli cell that is normally haploid but also contains an extra F plasmid carrying a second copy of the lac operon, used to study operon mutations.

Cis-acting mutations

Mutations that affect the expression of genes only on the same DNA molecule on which they reside (e.g., operator, promoter mutations).

Trans-acting mutations

Mutations that affect the expression of genes located on the same or different DNA molecules, often by producing a diffusable product like a protein (e.g., lacI protein).

Permease (lacY product)

An enzyme encoded by lacY that actively transports lactose into the E. coli cell.

β-Galactosidase (lacZ product)

An enzyme encoded by lacZ that breaks lactose into galactose and glucose, and also converts lactose into allolactose.

Allolactose

A compound formed from lactose by β-galactosidase that acts as the inducer for the lac operon, binding to the repressor protein.

Transacetylase (lacA product)

An enzyme encoded by lacA that transfers acetyl groups, though its specific role in lactose metabolism is less understood.

Repressor protein (LacI protein)

A protein continuously synthesized by the lacI gene that binds to the operator (lacO) in the absence of lactose, blocking RNA polymerase from transcribing the lac operon.

Inducer

A molecule (allolactose in the lac operon) that binds to a repressor protein, causing a conformational change that prevents the repressor from binding to the operator, thus inducing transcription.

Catabolite Repression (in E. coli)

A phenomenon where the presence of glucose in the growth medium preferentially shuts down the utilization of other sugars, including lactose, until glucose is depleted.

cAMP-CAP complex

An activator of transcription formed when cAMP binds to CAP, which then binds just upstream of the lacP promoter to enhance RNA polymerase activity and increase transcription rate.

lacI- mutation

A regulatory gene mutation that results in a nonfunctional repressor protein, leading to constitutive (always 'ON') transcription of the lac operon structural genes.

lacOC mutation

An operator mutation that prevents the repressor from binding to the operator sequence, resulting in constitutive (always 'ON') transcription of the lac operon.

lacIS mutation (Super-repressor)

A regulatory gene mutation that produces a repressor protein unable to bind the inducer (allolactose), causing it to always bind to the operator and permanently repress transcription, even in the presence of lactose.

lacP- mutation

A promoter mutation that impairs RNA polymerase binding, leading to a lack of transcription for the structural genes of the lac operon, regardless of lactose presence.