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.