6. Gene Structure and the Trp Operon

• Gene


  Segment of DNA that codes for a specific protein
• Structural genes


  Responsible for coding for proteins involved in the structure or function of a cell (e.g. enzymes, transport proteins, receptors, or peptide hormones)
• Regulatory genes


  Produce regulatory proteins, such as repressor or activator proteins, that repress or activate transcription and the expression of structural genes
• Eukaryotic genes


  • Promoter (TATAAA box) - region where RNA polymerase can bind
  • Introns - non-coding regions removed in RNA processing
  • Exons - coding sequences that code for the functional protein
  • Termination sequence - signals the end of transcription
• In eukaryotic cells, the mRNA is made in entirety first, then leaves the nucleus and is translated by ribosomes
• Prokaryotic genes


  • Structural genes that share a common purpose are grouped together and under the control of the same promoter and operator
  • This is called an operon
• In prokaryotes, ribosomes begin translating the mRNA straight away while RNA polymerase is still transcribing the DNA sequence
• Operon


  A cluster of genes that are transcribed together to give a single messenger RNA (mRNA) molecule, which therefore encodes multiple proteins
• Regulatory genes and repressor proteins


  • Operons have a regulatory gene that produces a repressor protein that can bind to the operator and stop transcription
• trp Operon


  A specific operon found in prokaryotes that is responsible for the production of the amino acid tryptophan
• The trp operon contains 5 structural genes (trpE, trpD, trpC, trpB and trpA) that encode enzymes that catalyse the production of tryptophan
• Tryptophan


  The rarest of the 20 amino acids, it is non-polar and humans cannot synthesise their own tryptophan
• Tryptophan codon (mRNA): ACC
• Promoter


  A nucleotide sequence that enables a gene to be transcribed, where RNA polymerase will bind to begin transcription
• Operator


  A segment of DNA to which a repressor protein can bind to control the expression of the structural genes
• Repression


  1. Prevents transcription from occurring based on the level of free floating tryptophan in the cell

  2. If low tryptophan, transcription and translation occurs and more tryptophan is made

  3. If high tryptophan, transcription does not begin and no translation occurs so no tryptophan is made
• Regulatory gene and trp Operon

  • The regulatory gene produces a repressor protein that can bind to the operator and control transcription of the trp structural genes
• High free floating tryptophan in cell cytosol


  Repressor protein binds to tryptophan, undergoes a conformational change, and can then bind to the operator to block transcription
• Low free floating tryptophan in cell cytosol


  Repressor protein does not bind to tryptophan, so it cannot bind to the operator, allowing transcription to occur
• Attenuation


  A process that prevents transcription from completing, rather than preventing it from happening (like repression)
• Attenuation of trp operon

  • Occurs when there are high levels of tryptophan attached to tRNA molecules but none that are free floating
  • Involves a leader region of the operon that contains codons for tryptophan and a stop codon
• Attenuation - Low tRNA trp levels

  1. Ribosome stalls at the tryptophan codons in the leader region, allowing regions 2 and 3 to form an anti-terminator hairpin, allowing transcription to continue

  2. RNA polymerase remains attached to the mRNA
• Attenuation - High tRNA trp levels

  Ribosome rapidly finishes translating the leader region, allowing regions 3 and 4 to form a termination hairpin, causing RNA polymerase and ribosome to be released and transcription of the operon to stop
• If there is no tryptophan present, either free in the cytoplasm or bound to tRNA, then transcription and translation of the operon occurs to make tryptophan
• The logic of attenuation is the same as regulation by the trp repressor - high levels of tryptophan shut down expression of the operon, as the cell does not need to make more biosynthetic enzymes
• The trp operon is expressed (turned "on") when trp levels are low and repressed/attenuated (turned "off") when trp levels are high