Control of Gene Expression and The Lac Operon

Vocabulary flashcards for Chapter 12.1-12.2 on control of gene expression in prokaryotes, with a focus on operons and the lac operon.

Polycistronic unit

In prokaryotes, one mRNA that encodes for more than one protein. Genes encoding products that act in the same pathway can be found in the same polycistron and are transcribed together.

Operon

A stretch of DNA containing a cluster of genes that are transcribed together from a single promoter (polycistronic).

Cistron

An mRNA that encodes one protein.

Promoter

A gene sequence necessary for transcription to initiate via binding of RNA polymerase.

Structural genes

Genes within an operon that encode proteins involved in a common process and are important for cell function.

Operator

A DNA sequence downstream of the promoter to which a regulator protein can bind and prevent movement of RNA polymerase, blocking transcription of structural genes. It overlaps with the 3' end of the promoter and sometimes the 5' end of the first structural gene.

Regulator protein

A protein encoded by a regulator gene that binds the operator and controls transcription of the structural genes. It is not encoded within the operon itself.

Repressor

A type of regulator protein that binds to the operator and blocks RNA polymerase movement, thereby blocking transcription of structural genes.

Activator

A type of regulator protein that activates transcription by binding upstream of the operator and facilitating RNA polymerase binding.

Positive Control (Operons)

A type of operon control where the regulator protein binds to the operon and is required to activate transcription; the protein is an activator. If the regulatory gene is mutated (no function), the operon is OFF.

Negative Control (Operons)

A type of operon control where the regulator protein binds to the operon and represses transcription; the protein is a repressor. If the regulatory gene is mutated (no function), the operon is ON.

Inducible Operon

An operon that is normally OFF and can be induced or turned ON for transcription, often in the presence of a specific metabolite (e.g., lactose in the lac operon).

Repressible Operon

An operon that is normally ON and can be repressed or turned OFF for transcription, often in the presence of a specific metabolite (e.g., Trp in the trp operon).

Negative inducible operon

Normally OFF due to an active repressor bound to the operator. A substrate acts as an inducer by binding to the repressor, making it inactive and releasing it from the operator, turning the operon ON.

Negative repressible operon

Normally ON because an inactive repressor is present and not bound to the operator. A product acts as a co-repressor that binds to the repressor, making it active and able to bind to the operator, turning the operon OFF.

Positive inducible operon

Normally OFF due to an inactive activator protein. A substrate acts as an inducer by binding to the inactive activator, making it active and able to bind to an activator sequence, which induces transcription and turns the operon ON.

Positive repressible operon

Normally ON because an active activator is bound to the activator sequence, inducing transcription. A product acts as a repressor that binds to the activator, making it inactive and unable to bind, turning the operon OFF.

Lac Operon

The first operon characterized, controlling lactose metabolism in E. coli, responsible for producing enzymes needed to convert lactose into glucose and galactose. It is under both negative and positive control.

lacZ

A structural gene within the lac operon that encodes β-galactosidase.

β-galactosidase

An enzyme encoded by lacZ that breaks down lactose into glucose and galactose.

lacY

A structural gene within the lac operon that encodes permease.

Permease

A protein encoded by lacY that facilitates the entry of lactose into the cell.

lacA

A structural gene within the lac operon that encodes transacetylase.

Transacetylase

An enzyme encoded by lacA that transfers acetyl groups.

lacP

The promoter region of the lac operon, a DNA sequence where RNA polymerase binds.

lacO

The operator region of the lac operon, a DNA sequence where the repressor protein binds.

lacI

The repressor-encoding gene (regulatory gene) for the lac operon, which produces the repressor protein that binds lacO.

Allolactose

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