Regulation of Gene Expression in Prokaryotes

Comprehensive vocabulary flashcards covering prokaryotic gene regulation, including inducible and repressible systems, the lac and trp operons, catabolite repression, attenuation, riboswitches, and regulatory sRNAs.

Constitutive Enzymes

Enzymes whose synthesis is not regulated and is therefore continuous regardless of environmental conditions.

Negative Control

A mode of gene regulation in which genetic expression occurs continuously unless it is shut off by a regulator molecule (normally ON).

Positive Control

A mode of gene regulation in which transcription occurs only when a regulator molecule directly stimulates RNA production (normally OFF).

Operon

A cluster of contiguous genes governed by a single promoter that functions as a coordinated unit of transcriptional regulation.

$\beta$-Galactosidase

An enzyme encoded by the lacZ gene that catalyzes the enzymatic breakdown of lactose into galactose and glucose, and also converts lactose into allolactose.

Galactoside Permease

A membrane-bound protein encoded by the lacY gene of the lac operon that facilitates the active entry and transport of lactose into the bacterial cell.

Transacetylase

An enzyme encoded by the lacA gene of the lac operon whose physiological role is the removal of toxic by-products resulting from lactose digestion.

Allolactose

An isomer of lactose generated by $\beta$-galactosidase that serves as the actual physiological inducer by binding to the repressor protein and triggering induction of the lac operon.

$lacI^-$ Mutant

A constitutive mutant in which the operator-binding domain of the repressor protein is altered, preventing repressor binding to the operator and causing continuous transcription in the absence of lactose.

$lacO^c$ Mutant

A constitutive cis-acting mutant in which the nucleotide sequence of the operator region is altered, preventing wild-type repressor binding and causing continuous structural gene transcription.

$lacI^s$ Mutant

A mutant repressor allele with an altered lactose-binding domain that fails to bind the inducer, causing the repressor to remain permanently bound to the operator and resulting in constitutive repression.

Catabolite-Activating Protein (CAP)

A regulatory protein exerting positive control over the lac operon; upon binding cAMP, it attaches to the CAP-binding site and facilitates RNA polymerase binding at the promoter to drive transcription.

Cyclic Adenosine Monophosphate (cAMP)

A signaling nucleotide generated from ATP by adenylyl cyclase that binds to CAP to induce an allosteric transition necessary for CAP to bind the lac promoter.

Catabolite Repression

The repression of alternative catabolic operons (such as the lac operon) caused by the presence of glucose, which inhibits adenylyl cyclase, lowers cAMP levels, and prevents efficient CAP binding.

The lac Operon Paradox

The apparent contradiction that permease is required for lactose to enter the cell, yet permease expression requires lactose induction; resolved by the fact that the repressor spontaneously and occasionally dissociates from the operator, allowing basal transcription.

Repression Loop

A loop formed in lac operon DNA when the repressor binds simultaneously to the primary operator ($O_1$) and an auxiliary operator ($O_3$), physically preventing RNA polymerase access to the promoter.

Tryptophan (trp) Operon

A repressible anabolic gene system in E. coli consisting of five contiguous structural genes (trpE, trpD, trpC, trpB, trpA) whose enzymes synthesize the amino acid tryptophan.

Corepressor

A small metabolite (such as tryptophan in the trp operon) that binds to an inactive repressor protein, triggering an allosteric transition that allows the repressor-corepressor complex to bind the operator and block transcription.

Attenuation

A second regulatory mechanism in the trp operon where transcription initiates normally but terminates early within a $5'$ leader sequence of approximately $140\,\text{bp}$ in response to high tryptophan levels.

Antiterminator Hairpin (2–3 Stem-Loop)

An mRNA secondary structure that forms between leader regions 2 and 3 when the translating ribosome stalls at tandem tryptophan codons due to low charged $\text{tRNA}^{\text{Trp}}$, allowing transcription of trp structural genes to proceed.

Terminator Hairpin (3–4 Stem-Loop)

A Rho-independent transcription terminator hairpin formed between regions 3 and 4 of the trp leader mRNA when charged $\text{tRNA}^{\text{Trp}}$ is abundant, causing premature termination of transcription.

Riboswitch

A regulatory domain in mRNA secondary structure that directly binds a small ligand to alter conformation, consisting of an aptamer domain that binds the ligand and an expression platform that adopts a terminator or antiterminator structure.

Bacterial Small Noncoding RNAs (sRNAs)

Short noncoding RNA molecules that regulate bacterial gene expression by base pairing with mRNAs, either inhibiting translation by masking the ribosome-binding site (RBS) or promoting translation by unmasking an otherwise folded RBS.