Genetic Regulation in Bacteria

Announcements

• Exam III starts tomorrow.
• Today’s post-lecture assignment is due at 1:00 PM on Thursday.
• No class or Student Hours on Friday.
• Student Hours today from 4:00-5:30 in 124 Burrill Hall.
• TA Help Session tomorrow from 11:00-Noon (MCB Learning Center).

The Cost of Making Proteins

• The lecture discusses the energetic cost of protein synthesis using the lacZ gene as an example.
• lacZ gene:
  • Approximately 3,000 nucleotides (nt) long.
  • It produces a messenger RNA (mRNA) that is translated by a polysome to yield a 1,000 amino acid (AA) protein.
• Cost analysis:
  • The rate of translation is about 20 amino acids per second.
  • Therefore, a 1,000 AA protein takes 1,000 \text{ AA} \cdot (1 \text{ sec} / 20 \text{ AA}) = 50 \text{ sec} to synthesize.
  • The energy cost is approximately 1 ATP equivalent per base, totaling 3,000 ATP for the lacZ mRNA.
  • An additional 3,000 ATP are required for the amino acids themselves.
  • The total ATP cost for a single copy of the protein is 6,000 ATP.
  • The rate of ATP consumption is 6,000 \text{ ATP} / 50 \text{ sec} = 120 \text{ ATP/sec}.

Regulation of Gene Expression

• Expressing all genes at all times is energetically too expensive for a cell.
• Cells have the ability to turn genes on and off as needed.
  • E. coli can use various sugars (e.g., glucose, lactose, arabinose, maltose) as food sources.
  • Each sugar requires a different set of enzymes for metabolism.
  • It is inefficient to produce enzymes for metabolizing lactose when no lactose is available.

Gene Regulation and Metabolic Adjustment

• Many genes are regulated, being turned on and off in response to changing cellular needs.
• The regulation of gene expression allows cells to adjust their metabolism to achieve maximum growth in a given environment.
• Constitutive Genes:
  • Some genes are needed at all times and are not regulated.
  • These genes are expressed constitutively, except during M-phase when transcription ceases broadly.

Points of Regulation

• Regulation can occur at various stages:
  • Transcription initiation
  • Transcript stability
  • Translation initiation
  • Rate of translation
  • Polypeptide stability
  • Activity of the protein

Trade-offs in Regulation

• Regulation can occur at every step of gene expression
• There is a tradeoff between energy savings and the speed at which the change takes effect.
  • Altering protein activity provides an immediate change but does not recover the ATP spent in creating the protein.
• Regulation most frequently occurs at the level of transcription initiation, balancing energy savings and response speed.

Lactose Operon (lac operon)

• An example of regulation at the level of transcription initiation in E. coli.
• Operons: Multiple protein-coding regions under the control of a single promoter, transcribed as a polycistronic mRNA.
  • Each protein-coding region in the mRNA has its own ribosome binding site (RBS) and stop codon.
  • β-galactosidase:
    • One of the protein products of the lac operon.
    • Its expression is highly regulated:
      • Less than 3 molecules per cell in the absence of lactose.
      • Greater than 3,000 molecules per cell when lactose is present.

Function of β-galactosidase and lac permease

• (The specific functions of β-galactosidase and lac permease are not detailed in this segment of the transcript.)

Regulation of the lac operon by Lactose and Glucose

• The expression of the lac operon is regulated by both lactose and glucose.
• Growth of E. coli on both glucose and lactose:
  • The expression of β-galactosidase is affected by the presence of both glucose and lactose.
• Two key questions:
  1. How does lactose induce β-gal expression?
  2. How does glucose reduce β-gal expression?