Biol 3130 Lecture 10: Control of Gene Expression in Prokaryotes: The Lac Operon

Flashcards covering the control of gene expression in prokaryotes, specifically focusing on the Lac Operon in E. coli, its components, regulation mechanisms, and key historical discoveries.

Lac Operon

A genetic regulatory system in E. coli that controls the metabolism of lactose.

Permease (lacY gene product)

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

β-galactosidase (lacZ gene product)

An enzyme that breaks down lactose into galactose and glucose, and also converts lactose into allolactose.

Allolactose

A related compound formed from lactose by β-galactosidase, which acts as the inducer for the lac operon.

lacA gene

Encodes transacetylase, an enzyme that transfers acetyl groups.

lacP (Operon Promoter)

A regulatory DNA sequence that serves as the RNA Polymerase binding site for the lac operon.

lacO (Operon Operator)

A regulatory DNA sequence where the repressor protein binds to block transcription of the lac operon.

lacI gene

Encodes the repressor protein, which binds to the operator and blocks RNA Polymerase binding to the promoter.

Negative Control of Lac Operon

In the absence of lactose, the lacI repressor protein binds to the operator, preventing (or mostly turning OFF) transcription.

Inducer of Lac Operon

Allolactose, which binds to the repressor protein causing a conformational change that releases it from the operator, inducing transcription.

LacI Repressor Binding

Binds to the lacO region as a tetramer (four identical polypeptides), each with a DNA binding domain and a regulatory domain.

Catabolite Repression

The phenomenon in E. coli where glucose presence shuts down the import and conversion of other simple sugars, even if present, as glucose is the preferred food source.

Positive Control of Lac Operon

Accomplished by Catabolite Activator Protein (CAP), which enhances the transcription rate of the lac operon.

cAMP (Cyclic AMP)

High levels activate CAP; cAMP binds to CAP to form the cAMP-CAP complex, which acts as an activator of lac operon transcription.

cAMP-CAP Complex

An activator that binds upstream of the lac promoter (lacP), enhancing the initiation of transcription by increasing RNA polymerase affinity.

Jacob and Monod

Scientists who devised an elegant system using partial diploids to study the functions of the different components of the lac operon, earning them the Nobel Prize in 1965.

Partial Diploid

An E. coli cell that has its full bacterial chromosome (haploid) plus an extra small circular piece of DNA (F plasmid) containing lac operon sequences, effectively giving it two copies of the lac operon.

Haploid

A cell containing one copy of the genome per cell, typical for most prokaryotes and germ cells of eukaryotes.

Diploid

A cell containing two copies of the genome per cell, typical for somatic cells of eukaryotes.

Cis-acting Mutations

Mutations that affect the expression of genes only on the same DNA molecule where 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 because their protein product can diffuse and act elsewhere (e.g., lacI repressor protein).

lacI- Mutation

A mutation in the regulatory gene (lacI) leading to an inactive repressor, resulting in constitutive (always 'ON') transcription of the structural genes, regardless of lactose presence.

lacOc Mutation

A constitutive mutation in the lacO operator sequence where the repressor can no longer bind, causing the lac operon to be always transcribed.

lacIS (Super-repressor) Mutation

A mutation resulting in a super-repressor protein that cannot bind the inducer (allolactose) and therefore continuously binds to the operator, repressing transcription even in the presence of lactose.