Prokaryotic Gene Regulation and CRISPR-Cas Immunity in Bacteria Notes

Prokaryotic Gene Regulation in Bacteria

Learning Goals

  • Understand types of genetic regulation.

  • Study regulation of the lac operon.

  • Examine the roles of lactose and glucose.

  • Explore CRISPR-Cas immunity in bacteria.

Benefits of Differential Gene Expression

  • Cells require varying amounts of gene products.

    • Example: Cells need many ribosomal proteins but fewer for mismatch repair proteins.

  • This regulation saves energy by producing proteins only when needed.

    • Example: Enzymes for lactose digestion are made only when lactose is the sole energy source.

Types of Gene Regulation

  • Constitutive: Gene is always expressed.

  • Inducible: Gene expression is in response to specific stimuli.

  • Repressible: Gene expression is suppressed in response to specific stimuli.

Proteins That Control Gene Regulation

  • Activator: A protein that increases transcription by binding to DNA (positive control).

  • Repressor: A protein that decreases transcription by binding to DNA (negative control).

Operons in Prokaryotes

  • Genes grouped together in bacterial chromosomes, sharing one promoter and regulatory region.

  • Commonly associated with processes saving energy and space.

  • Operons are unique to bacteria; eukaryotes do not have operons.

Lac Operon in E. coli

  • Components:

    • LacZ: β-galactosidase, breaks down lactose.

    • LacY: Permease, helps lactose enter cells.

    • LacA: Transacetylase, handles byproducts of lactose metabolism.

    • LacI: Repressor molecule, represses lac operon expression.

Inducible Nature of Lac Operon

  • Lac operon is typically not expressed without lactose.

  • The LacI repressor binds to the operator (O) preventing gene expression.

  • When lactose is present, it binds to LacI, causing it to release from O, allowing transcription.

CAP and Lac Operon Regulation

  • CAP (Catabolite Activator Protein): Induces lac gene expression when glucose is absent.

  • cAMP (cyclic AMP) is formed when glucose is low, binding to CAP enables it to bind to the lac operon promoter.

  • When glucose is present, cAMP is low, inhibiting CAP binding and reducing lac gene expression.

Summary of Lac Operon Expression Conditions

  • Lactose Only: High expression; repressor inactive; CAP may be active based on glucose.

  • Glucose Only: Low expression; repressor inactive; CAP inactive.

  • Glucose and Lactose: Low expression; no functional CAP binding; repressor inactive.

CRISPR-Cas Immunity in Bacteria

  • Adaptive Immunity: Developed through past exposures to pathogens (like antibodies from vaccines).

  • CRISPR locus: Contains viral DNA between repeat sequences in spacer regions.

  • Cas proteins: Endonuclease proteins that cut DNA in the presence of crRNA.

CRISPR-Cas Process**

  1. Acquisition: Viral DNA is cut and incorporated as spacers into the bacterial chromosome.

  2. Synthesis of crRNA: Transcription of CRISPR locus generates crRNA that binds to Cas proteins.

  3. Target Interference: If the virus re-invades, crRNA binds to viral DNA, and Cas cuts it to prevent infection.

Key Questions about CRISPR-Cas

  • Spacer Sequences in CRISPR:

    • Answer: B. Sequences from invading viruses or plasmids.

  • Implications of Spacer Mutation:

    • Answer: B. Yes, the crRNA from that spacer would no longer bind to the phage DNA to target degradation by Cas.