Prokaryotic Gene Expression and the Operon Model

Central Dogma

DNA à mRNA à Protein

Human genome size

3.23 Gbp (haploid), 22,000 genes

Protein-coding DNA percentage

Only 1.5% of DNA codes for proteins

Regulatory sequences percentage

Much of the remaining 97% is regulatory sequences

Differential gene expression

The expression of different sets of genes by cells with the same genome

Specific transcription factors

Each gene requires particular transcription factors; different cells have different transcription factors.

Gene 1 product

Optimizes eyesight in air

Gene 2 product

Optimizes eyesight in water

Gene 3 product

Required in all eye cells

Negative regulation

Gene normally transcribed; regulation decreases (turns off) expression

Repressible gene

A gene that can be turned off by negative regulation

Positive regulation

Gene not normally transcribed; regulator turns on expression

Inducible gene

A gene that can be turned on by positive regulation

Prokaryotic transcription and translation

Occur simultaneously in the cytoplasm, with regulation at the transcriptional level.

Eukaryotic gene expression regulation

Regulated during transcription and RNA processing in the nucleus, and during protein translation in the cytoplasm.

Post-translational modifications

Further regulation that may occur after protein synthesis.

Constitutive genes

Genes that are 'ON' all the time, such as those involved in the glycolysis pathway.

Repressible Systems

Systems that downregulate mRNA transcription, degrade mRNA, decrease translation, degrade protein, and inhibit protein.

Constitutive Genes

Genes that are 'ON' all the time.

Inducible Systems

Genes that are 'OFF' most of the time and are turned 'ON' only when needed.

Metabolic Pathway Regulation

Two ways to regulate a metabolic pathway: 1. Regulate expression (transcription or translation); less energy is wasted. 2. Regulate the protein itself after it is made.

Allosteric Regulation

Regulation that involves the regulation of protein synthesis itself.

Operon

A gene cluster with a single promoter, two or more structural genes that encode protein/polypeptide, and an operator that is a regulatory sequence recognized by activator proteins.

Lac Operon

Regulates 3 genes needed for lactose metabolism.

LacI Gene

Adjacent to the operon, synthesizes a repressor that binds to lac O (operator), and is always on (constitutive).

Inducible System

The lac operon is an inducible system that is induced by the presence of lactose (specifically allolactose, an isomer of lactose).

Repressor Binding

The repressor can bind to the operator or the inducer (lactose). If lactose is absent, the repressor binds to the operator.

Lactose Presence Effect

If lactose is present, the repressor binds to lactose, which is an inducer.

Lac Operon Shutdown

When all the lactose is broken down, it is removed from the repressor and the lac operon shuts off.

Glucose Availability

If a bacterial cell has plenty of glucose available, it should not turn on the lac operon even if lactose is present.

Positive control of the lac operon

Gene regulation with catabolite activator protein (CAP) and cyclic AMP (cAMP): example of positive control.

CAP

Also known as cyclic AMP response protein (CRP).

cAMP

Cyclic adenosine monophosphate, a common chemical signal that has a diversity of roles, including as a second messenger in many eukaryotic cells, and as a regulator of some bacterial operons.

Low glucose effect

Low glucose triggers signaling, producing cAMP (cell stress).

cAMP and CAP complex

When cAMP binds to CAP/CRP, the complex binds to the CAP site near the lac promoter.

Transcription enhancement

Enhanced RNA polymerase binding which increases transcription of the lac operon.

Lactose presence effect

Lactose binds to repressor, preventing it from binding to the operator, thus turning the lac operon 'ON'.

Polycistronic

Operons are polycistronic: one mRNA codes for multiple proteins (enzymes).

Prokaryotic gene expression

Operon model, polycistronic, single promoter controls expression of multiple proteins, coordinated response.

Eukaryotic gene expression

Monocistronic, scattered genes on multiple chromosomes.

Inducible systems

Metabolic substrate (inducer) interacts with a regulatory protein (repressor); repressor does not bind and allows transcription.

Repressible systems

Metabolic substrate (corepressor) binds to regulatory protein (repressor), which then binds to the operator and blocks transcription.

trp operon

The trp operon is a repressible system.

Operator

In bacterial DNA, a sequence of nucleotides near the start of an operon to which an active repressor can attach, preventing RNA polymerase from attaching to the promoter.

Operon

A unit of genetic function found in bacteria, consisting of a promoter, an operator, and a coordinately regulated cluster of genes whose production function in a common pathway.

Repressor

A protein that inhibits gene transcription by binding to the DNA in or near the promoter.

Regulatory gene

A gene that codes for a protein, such as a repressor, that controls the transcription of another gene or group of genes.

Corepressor

A small molecule that binds to a bacterial repressor protein and changes the protein's shape, allowing it to bind to the operator and switch an operon off.

Inducer

A specific small molecule that binds to a bacterial repressor protein and changes the repressor's shape so that it cannot bind to an operator, thus switching an operon on.

Activator

A protein that binds to DNA and stimulates gene transcription.