Lac and Trp Operons: Regulation and Mechanisms

Lac Operon

An inducible system that allows E. coli to metabolize lactose only when lactose is present and glucose is scarce.

Structural Genes of Lac Operon

Includes lacZ, lacY, and lacA, which encode enzymes for lactose metabolism.

lacZ

Encodes β-galactosidase, which breaks lactose into glucose and galactose.

lacY

Encodes lactose permease, a membrane protein that transports lactose into the cell.

lacA

Encodes thiogalactoside transacetylase, thought to detoxify byproducts.

Regulatory Elements of Lac Operon

Includes promoter (P), operator (O), lacI, and CAP binding site.

Promoter (P)

RNA polymerase binds here to initiate transcription.

Operator (O)

DNA sequence where the Lac repressor (LacI) binds.

lacI

A separate gene that codes for the Lac repressor protein, which is constitutively expressed.

CAP binding site

Upstream of promoter; binds CAP-cAMP to enhance transcription.

Regulation Mechanism of Lac Operon

Describes how the presence or absence of lactose and glucose affects transcription.

No Lactose Present

LacI repressor binds to the operator, blocking RNA polymerase from transcribing lacZYA.

Lactose Present

Allolactose binds the Lac repressor, causing it to detach from the operator, allowing transcription of lacZYA.

Presence of Glucose

When glucose is abundant, low cAMP levels prevent CAP from binding DNA, reducing transcription.

Lac Operon Summary

A table summarizing the conditions, repressor binding, CAP binding, and transcription levels.

Trp Operon

A repressible system that allows E. coli to synthesize tryptophan when it's not available in the environment.

Structural Genes of Trp Operon

Includes trpE, trpD, trpC, trpB, and trpA, which code for enzymes needed in the biosynthesis of tryptophan.

Regulatory Elements of Trp Operon

Includes promoter (P), operator (O), leader sequence (trpL), and trpR.

Leader Sequence (trpL)

Contains attenuator region that regulates transcription via RNA secondary structures.

trpR

Encodes Trp repressor, which is inactive without tryptophan.

Repression in Trp Operon

No tryptophan means the repressor is inactive, allowing transcription; high tryptophan activates the repressor, blocking transcription.

Attenuation in Trp Operon

Occurs after transcription begins but before full mRNA is made, depending on the trpL leader sequence.

Ribosome Stalling in Trp Operon

If tryptophan is low, ribosome stalls at trp codons, allowing transcription to continue; if high, transcription is terminated.

Trp Operon Summary

A table summarizing the conditions, repressor binding, attenuation, and transcription levels.

Inducible Pathway

A pathway that is activated in response to the presence of a specific molecule.

Repressible Pathway

A pathway that is inhibited in the presence of a specific molecule.

Default State of Lac Operon

OFF

Default State of Trp Operon

ON

Key Effector Molecule of Lac Operon

Allolactose (inducer)

Key Effector Molecule of Trp Operon

Tryptophan (corepressor)

Regulation Method of Lac Operon

Repressor + CAP-cAMP

Regulation Method of Trp Operon

Repressor + Attenuation

DNA Methylation

Addition of methyl groups to cytosine bases in CpG islands, typically represses gene expression by blocking transcription factor binding.

Histone Acetylation

Modification by HATs that relaxes chromatin and activates transcription.

Histone Deacetylation

Modification by HDACs that condenses chromatin and represses transcription.

Transcription Factors

Proteins that bind promoter or enhancer regions to activate or repress transcription.

Enhancers

Distal regulatory elements that increase transcription.

Silencers

Distal regulatory elements that repress transcription.

Mediator Complex

Complex that connects transcription factors with RNA polymerase II.

Alternative Splicing

Process that produces multiple proteins from the same gene.

mRNA Editing

Chemical modifications such as A-to-I editing.

mRNA Stability

AU-rich elements in 3' UTR affect degradation rates.

5' UTR Structures

Hairpins or upstream ORFs that can inhibit translation.

miRNAs

Small non-coding RNAs that bind mRNA and either degrade it or block translation via the RISC complex.

Point Mutations

Mutations that involve a change in a single nucleotide.

Silent Mutation

A point mutation that does not change the amino acid sequence.

Missense Mutation

A point mutation that results in a different amino acid.

Nonsense Mutation

A point mutation that creates a premature stop codon.

Frameshift Mutations

Mutations caused by insertions/deletions not in multiples of 3, resulting in a complete change of downstream amino acid sequence.

Chromosomal Mutations

Mutations that involve duplications, deletions, inversions, or translocations.

Mismatch Repair (MMR)

Mechanism that fixes replication errors not caught by proofreading.

Base Excision Repair (BER)

Mechanism that repairs non-bulky lesions such as deaminated bases.

Nucleotide Excision Repair (NER)

Mechanism that repairs bulky lesions such as thymine dimers.

Direct Repair

Mechanism that reverses damage directly without excision of bases.

Double-Strand Break Repair

Mechanism that includes homologous recombination and non-homologous end joining.