L3: Gene Expression and Operon Regulation Flashcards

Transcript Initiation and Regulation of Gene Expression

• Gene Expression Overview:

  • Gene expression is the process of transcribing a gene into messenger RNA ($\text{mRNA}$) and subsequently translating that $\text{mRNA}$ to synthesize a protein.

  • Gene expression is energetically expensive for the cell, making precise regulation and control essential.

  • Regulation can occur at numerous stages, but controlling the initiation of transcription is one of the primary regulatory checkpoints.

• Promoter Architecture:

  • $-35$ Sequence: A DNA sequence within the promoter that is recognized by RNA polymerase to initiate binding.

  • Prudenal Box: A sequence located at approximately $-10$ within the promoter where the replication/transcription bubble begins to form.

  • Operators: Specific DNA sequences situated within or near promoters that serve as binding sites for regulatory proteins to control when transcription starts.

• Classes of Regulatory Proteins:

  • Repressors: Regulatory proteins that inhibit or prevent transcription.

  • Mechanics: A repressor recognizes and binds to a specific operator sequence. Because a given operator sequence is typically present in only $1$, $2$, or $3$ promoters across the entire chromosome, a repressor controls a very specific subset of genes.

  • Mode of Inhibition: Upon binding the operator, the repressor physically blocks the promoter so that RNA polymerase cannot bind, turning the gene off.

  • Small Molecule Interactions:

    • Inducer-Controlled Repressors: The repressor remains bound to the promoter until a specific small molecule, termed an inducer, binds to it. Inducer binding causes the repressor to release the operator, exposing the promoter so RNA polymerase can bind and induce transcription.

    • Co-repressor-Controlled Repressors: The repressor cannot bind the operator or block transcription on its own. It requires a helper small molecule called a co-repressor. When both the repressor and co-repressor are present, they bind the operator together to block expression. If the co-repressor concentration decreases and it leaves, the repressor detaches, exposing the promoter for transcription.

  • Activators: Regulatory proteins that activate or stimulate transcription.

  • Mechanics: Activators bind to specific operator sequences within promoters to assist RNA polymerase in locating and binding the promoter.

  • Promoter Context: Genes regulated by activators typically feature weak promoters whose $-35$ sequences are not readily recognized by RNA polymerase without assistance.

  • Small Molecule Interactions:

    • Inhibitor-Controlled Activators: The activator functions independently to promote transcription unless a specific inhibitor molecule binds to it, which halts its activating function.

    • Inducer-Controlled Activators: The activator cannot bind DNA or activate transcription unless it is bound to a specific helper molecule, which in this context is also termed an inducer. The inducer causes the activator to latch onto the promoter sequence.

Operon Organization and Specific Bacterial Operons

• Operon Structure in Bacteria:

  • Operons are multi-gene regulatory units found in bacteria (absent in eukaryotes).

  • An operon consists of a single promoter controlling multiple transcribed regions (coding regions) followed by a single terminator.

  • Flashing a single regulatory switch enables the simultaneous, co-regulated expression of all enzymes encoded within the operon in equal stoichiometric amounts.

• The trp Operon:

  • Structural Components: Consists of $1$ promoter, $5$ distinct transcribed regions, and $1$ single terminator.

  • Function: Produces $5$ distinct enzymes required for the biosynthesis of the amino acid tryptophan.

  • Regulation Type: Regulated by the trp repressor, which functions in tandem with a co-repressor.

  • Co-repressor Identity: The amino acid tryptophan itself acts as the co-repressor.

  • Regulatory Mechanism:

  • When free tryptophan is abundant in the cytoplasm, excess tryptophan binds to the trp repressor as a co-repressor.

  • The repressor-co-repressor complex binds the trp operator, physically blocking RNA polymerase and turning off operon expression.

  • As the cell consumes tryptophan for protein synthesis, cytoplasmic tryptophan levels fall until no co-repressor is available.

  • Deprived of co-repressor, the trp repressor detaches from the operator, allowing RNA polymerase to transcribe the operon, synthesize $\text{mRNA}$, and produce the $5$ enzymes to manufacture more tryptophan.

  • Accumulation of synthesized tryptophan restores co-repressor binding, causing the trp repressor to rebind the operator and halt transcription.

  • Feedback Inhibition: The molecular product of the operon's enzymatic pathway directly inhibits the transcription of the operon.

• The lac Operon:

  • Structural Components: Consists of $1$ single promoter controlling $3$ transcribed coding regions.

  • Function: Encodes enzymes that allow the bacterial cell to catabolize the sugar lactose as an energy source.

  • Repressor Regulation: Controlled by the lac repressor protein.

  • Inducer Identity: Lactose itself acts as the inducer.

  • Regulatory Mechanism:

  • In the absence of lactose, the lac repressor remains bound to the promoter/operator region, blocking transcription to prevent wasting cellular energy on unused enzymes.

  • When lactose enters the cell, it acts as an inducer by binding to the lac repressor, causing the repressor to undergo a conformational change and detach from the promoter.

Dual Regulation of the lac Operon and Starvation Signaling

• Combinatorial Logic of the lac Operon:

  • Removal of the lac repressor by lactose is insufficient on its own to initiate robust transcription; the operon requires dual regulatory inputs involving both a repressor and an activator.

  • Bacteria prefer glucose over lactose as a primary energy source. As long as glucose is available, the cell utilizes glucose via glycolysis and the Krebs cycle and suppresses lactose utilization.

• Starvation Signaling Cascade:

  • When environmental glucose is exhausted, glycolysis and the Krebs cycle halt.

  • As cellular processes continue to consume energy without glucose, high-energy adenosine triphosphate ($\text{ATP}$) is converted to low-energy adenosine diphosphate ($\text{ADP}$).

  • To survive starvation, the cell begins cleaving phosphate groups off $\text{ADP}$ molecules as an emergency energy source, reducing $\text{ADP}$ (two phosphates) to adenosine monophosphate ($\text{AMP}$, one phosphate).

  • This emergency breakdown produces a specialized circular molecule called cyclic AMP ($\text{cAMP}$).

  • $\text{cAMP}$ acts as an internal cell-wide signal indicating severe glucose starvation and $\text{ATP}$ depletion.

• Metabolite Activating Protein (CAP) Activation:

  • CAP (Catabolite Activating Protein or Metabolite Activating Protein) is an activator protein that regulates the lac operon.

  • $\text{cAMP}$ functions as an inducer for CAP.

  • When $\text{cAMP}$ binds to CAP, CAP binds to a specific operator sequence near the lac promoter and assists RNA polymerase in recognizing the weak promoter.

• Dual Conditions for lac Operon Expression:

  1. Lactose must be present: Lactose binds the lac repressor, forcing it to detach from the promoter.

  2. Glucose must be absent: Absence of glucose elevates $\text{cAMP}$ levels, enabling $\text{cAMP}$ to bind CAP, which then binds the promoter to activate RNA polymerase.

  • If glucose is absent but lactose is also absent, the lac repressor remains physically bound to the promoter, preventing CAP and RNA polymerase from transcribing the operon regardless of $\text{cAMP}$ levels.

Structure of Messenger RNA and Translation Machinery

• mRNA Transcript Anatomy:

  • Transcription produces a single-stranded $\text{mRNA}$ molecule from double-stranded chromosomal DNA.

  • An $\text{mRNA}$ transcript contains three functional regions:

  1. $5'$ Untranslated Region ($5'$\,UTR): Non-coding region at the $5'$ end involved in ribosome recognition.

  2. Open Reading Frame (ORF): The middle coding sequence consisting of triplet codons that specify the amino acid sequence of the protein.

  3. $3'$ Untranslated Region ($3'$\,UTR): Non-coding region at the $3'$ end.

• Ribosome Binding Sequences:

  • Prokaryotes: Feature a specific sequence within the $5'$\,UTR called the Shine-Dalgarno sequence.

  • Eukaryotes: Feature a sequence within the $5'$\,UTR known as the Kozak box.

  • Function: Recognized by the ribosome to position itself properly on the $\text{mRNA}$ transcript to initiate translation.

• Ribosomal Structure and Functional Sites:

  • Ribosomes consist of a large subunit and a small subunit.

  • The $\text{mRNA}$ transcript is pulled through the interface between the two subunits from the $5'$ end to the $3'$ end.

  • The large subunit contains three functional pockets or sites:

  1. E site (Exit site)

  2. P site (Peptidyl site)

  3. A site (Aminoacyl site)

  • Spatial Dimensions: Each site is exactly $1$ codon wide ($3$ nucleotides wide) with no gaps between sites (e.g., E site covers nucleotides $1$, $2$, $3$; P site covers $4$, $5$, $6$; A site covers $7$, $8$, $9$).

• Transfer RNA (tRNA) Structure:

  • Single-stranded RNA molecule that folds back on itself into a stem-loop structure.

  • Stems: Double-stranded regions created by internal complementary base pairing.

  • Loops: Single-stranded regions created where base pairing does not occur.

  • $3'$ End: Attachment site where a specific amino acid is covalently bound.

  • Anticodon Loop: The bottom loop containing a triplet sequence of nucleotides called the anticodon, which base-pairs anti-parallel with matching codons on $\text{mRNA}$.

Step-by-Step Mechanics of Translation

• Initiation:

  • The ribosome binds the Shine-Dalgarno sequence or Kozak box in the $5'$\,UTR and feeds $\text{mRNA}$ through until the first codon of the ORF enters the ribosome.

  • Start Codon: The nucleotide sequence AUG serves as the universal start codon located at the beginning of the ORF.

  • Initiator tRNA: AUG recruits a specific tRNA carrying the complementary anticodon UAC.

  • First Amino Acid: The AUG start codon specifically codes for the amino acid methionine.

• Elongation Cycle:

  1. P Site Alignment: The AUG start codon and its paired methionine-tRNA align in the P site, leaving the A site empty and exposed to the second codon (e.g., sequence GGA).

  2. A Site Entry: An incoming tRNA with a complementary anticodon (e.g., CCU) and its attached amino acid enters the empty A site and base-pairs with the exposed codon.

  3. Peptide Bond Formation: The ribosome catalyzes a covalent peptide bond connecting the amino acid on the tRNA in the P site to the amino acid on the tRNA in the A site.

  4. Cleavage: The amino acid in the P site is enzymatically cleaved loose from its tRNA.

  5. Translocation: The ribosome shifts relative to the $\text{mRNA}$ by exactly $1$ codon ($3$ nucleotides) toward the $3'$ direction:

  • The uncharged tRNA moves into the E site.

  • The tRNA bearing the growing peptide chain shifts from the A site to the P site.

  • The A site becomes vacant, exposing the third codon.

  1. Ejection and Reloading: The uncharged tRNA in the E site is ejected into the cytoplasm, where cytoplasmic enzymes reload it with its corresponding amino acid for future rounds of translation.

  2. Repetition: The cycle repeats sequentially, elongating the amino acid chain.

• Termination:

  • Stop Codons: Translation continues until one of three stop codons enters the A site:

  • UAG

  • UAA

  • UGA

  • Absence of Matching tRNA: Stop codons do not correspond to any tRNA molecule, causing the translational machinery to pause at the empty A site.

  • Release Factors: Specialized proteins called release factors recognize the stalled ribosome at a stop codon.

  • Disassembly: Release factor causes the ribosome to cleave the completed polypeptide chain from the final tRNA, leading the protein to fold into its functional shape, the ribosomal subunits to fall off the $\text{mRNA}$, and $\text{mRNA}$ and tRNAs to be recycled.

The Genetic Code and Chemical Bonding

• Codon Metrics and Degeneracy:

  • $\text{mRNA}$ is composed of $4$ distinct nucleotides: Adenine (A), Uracil (U), Guanine (G), and Cytosine (C).

  • Nucleotides are read in groups of $3$ (codons), resulting in $4^3 = 64$ unique codon combinations.

  • Distribution of Codons:

  • $1$ start codon (AUG)

  • $3$ stop codons (UAG, UAA, UGA)

  • $60$ additional sense codons

  • There are $61$ sense codons that code for amino acids, corresponding to $61$ distinct tRNA molecules across the cell.

  • Because there are $61$ amino acid codons but only $20$ standard amino acids, the genetic code is redundant (degenerate).

  • Example of Redundancy: The amino acid leucine is specified by $6$ different codons in most organisms, requiring $6$ distinct tRNA species with different anticodons.

• Chemical Bonds in Protein Synthesis:

  • Amino Acid to tRNA Attachment: Joined by a standard covalent bond at the $3'$ end of the tRNA.

  • Peptide Bond: A specific type of covalent bond linking individual amino acids in a polypeptide chain.

  • Chemical Structure of a Peptide Bond: Formed between a carbon atom (bearing a double-bonded oxygen) of one amino acid's carboxyl group and a nitrogen atom (bearing a hydrogen) of the adjacent amino acid's amino group ($\text{C-N}$ covalent bond formed via shared electron pairs).

Questions & Discussion

• Question: Do the left and right loops of the tRNA stem-loop structure have specific functions?

  • Answer: In this context, no specific purpose is assigned to the side loops, unlike the bottom loop which contains the anticodon.

• Question: What types of bonds link amino acids together versus linking amino acids to tRNA?

  • Answer: All of these connections are standard covalent bonds formed by shared electron pairs. The covalent bond linking adjacent amino acids together is specifically termed a peptide bond due to the specific arrangement of carbon, oxygen, nitrogen, and hydrogen atoms surrounding the bond.

• Question: Does a bacterial cell produce regular AMP instead of cyclic AMP if it is not starving?

  • Answer: No. Cyclic AMP ($\text{cAMP}$) is specifically synthesized as an emergency signal when $\text{ATP}$ is depleted and the cell begins utilizing $\text{ADP}$ for energy during starvation.

• Question: Does lac operon expression strictly require both the presence of lactose and the absence of glucose?

  • Answer: Yes. Lactose must be present to remove the lac repressor from the promoter, and glucose must be absent to raise $\text{cAMP}$ levels so CAP can activate transcription. If lactose is absent, the lac repressor stays bound and physically blocks transcription regardless of CAP activation.