chapter 16

• Prokaryotic gene expression


  Regulated in operons
• Prokaryotic gene expression regulation


  • Regulatory proteins bind to promoters
  • Negative regulation - a repressor protein prevents transcription
  • Positive regulation - an activator protein stimulates transcription
• Prokaryotes make some proteins only when they are needed, through the mechanism of transcription regulation
• Regulation of metabolic pathways


  1. Allosteric regulation of enzyme-catalyzed reactions allows rapid fine-tuning

  2. Regulation of protein synthesis is slower but conserves energy and resources
• E. coli in the intestine must adjust quickly to changes in food supply
• Glucose is the easiest sugar for E. coli to catabolize to make ATP
• When glucose is unavailable, E. coli cells must use other food sources like lactose
• Inducers


  Compounds that stimulate protein synthesis
• Inducible proteins


  Proteins that are synthesized in response to inducers
• Constitutive proteins


  Proteins made all the time at a constant rate
• Structural genes


  Specify primary protein structure - the amino acid sequence
• Lac operon


  Gene cluster with a single promoter, containing three structural genes for lactose enzymes
• Operon


  Gene cluster with a single promoter, consisting of a promoter, two or more structural genes, and an operator
• Operator


  A short sequence between promoter and structural genes that binds to regulatory proteins
• Lac operon regulation


  1. Lactose absent - Repressor prevents binding of RNA polymerase to promoter, blocking transcription

  2. Lactose present - Lactose binds to repressor, changing its shape so it can no longer bind operator, allowing transcription
• Trp operon


  A repressible system - always on unless repressed by tryptophan (co-repressor)
• Inducible systems


  Substrate (inducer) interacts with a regulatory protein (repressor), preventing repressor from binding to operator and allowing transcription
• Repressible systems


  A product (co-repressor) binds to a regulatory protein, which then binds to the operator and blocks transcription
• Negative control


  Transcription is decreased in presence of a repressor protein
• Positive control


  An activator protein increases transcription
• Regulation of lac operon


  1. Relative levels of glucose and lactose determine amount of transcription

  2. Efficient transcription requires binding of an activator protein to the promoter
• Catabolite repression


  Gene regulation in which presence of a preferred energy source represses other catabolic pathways
• Prokaryotic promoters


  Have 2 recognition sequences, beginning 10 and 35 base pairs upstream of the transcription start site (-10 and -35 elements)
• Sigma factor


  An additional protein required for prokaryotic RNA polymerase to recognize both the -35 and -10 promoter sequences
• Sigma-70 factor


  Active most of the time and binds to recognition sequences of housekeeping genes
• Eukaryotic gene expression regulation


  • Nucleosome remodeling - histones must be selectively removed
  • Promoter region recognition - many transcription factors must be present
  • Pre-mRNA splicing and 3' PolyA tail length regulation
  • mRNA quality checks - unspliced introns catabolized
  • Regulated mRNA transport through cytoplasm
• Eukaryotic promoters


  Sequences near the 5' end of the coding region, including a TATA box and regulatory sequences recognized by transcription factors
• General transcription factors


  Assemble on the chromosome to allow RNA polymerase II to bind the promoter
• Enhancers


  Regulatory sequences that bind transcription factors to activate or increase transcription rate
• Silencers


  Regulatory sequences that bind transcription factors to repress transcription
• Mediator protein


  Binds to the basal transcription apparatus and causes DNA bending to bring regulatory sequences close to the promoter
• Helix-turn-helix DNA-binding motif


  A common structural motif in transcription factor domains that bind to DNA
• Cell differentiation


  1. Mediated by transcription factors that determine gene expression

  2. Transcription factors first inhibit cell division, then control other differentiation processes
• Eukaryotes coordinate expression of sets of genes by having common regulatory sequences that can be bound by the same transcription factors
• Coordinated gene expression in land plants


  • Genes for drought stress response proteins have a common stress response element (SRE) that is bound by a transcription factor to stimulate mRNA synthesis