Generalities for Transcriptional Regulation

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Generalities for Transcriptional Regulation

• Activators and Repressors Act in Concert
• Effector Binding Changes Protein Shape
• DNA Binding Occurs at Specific Sequences
• DNA Looping Common Feature

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Eukaryotes vs. Prokaryotes

• Complexity of Genome
• Multiple, Compartmentalized Chromosomes
• Transcription Separated from Translation
• Transcript Processing
• Multi-Cellular

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Transcriptional Regulation
Translational Regulation
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Eukaryotic RNA Polymerases

• RNA Polymerase I
• Ribosomal RNA Genes
• RNA Polymerase II
• Protein Coding Genes
• RNA Polymerase III
• Transfer RNA Genes

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Hsp 83 RNA Accumulation During Drosophila
Embryogenesis
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Enhancerscis-acting elements

• Act at a distance
• May be upstream or downstream
• Bind Activators and/or Repressors
• Can act on additional genes

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Fig. 16.5
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lacZ expression Pattern
-873.1621
-873.1009
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Activating Transcription Factors

• Great Diversity
• Protein Domains
• DNA Binding Domain
• Activator Domain
• DNA Binding Domain Motifs
• Helix-Turn-Helix (aka. Homeodomain)
• Zinc Finger
• Basic Leucine Zipper
• Helix-Loop-Helix

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Homeodomain (Helix-Turn-Helix)
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Zinc Fingers
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Steroid--Steroid Receptor Complex
Receptor
Hormone
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Basic Leucine Zipper (bZIP)
Fig. 16.7
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Myc/Max Helix Loop Helix
Fig. 16.10
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Fig. 16.8
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Fig. 16.11
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Chromatin Structure and Gene Expression

• Dampens Transcription to Low Levels
• Major Means of Keeping Expression Turned Off
• In contrast, prokaryotes utilize repressors
• Chromatin remodeling precedes activation
• Reflected by DNAase I hypersensitivity
• Heterochromatin Silences All Transcription
• Example X-inactivation and formation of Barr
  Bodies

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Sex Determination in Drosophila

• XX Females, XY Males
• Early Embryo Counts the X Chromosomes
• Sex Lethal (Sxl) Gene Critical Regulator
• Mutations in Sxl Result in NO Females!
• Sex Lethal Acts at the Level of Splicing
• RNA-Binding Splice Factor

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Early Embryo
Helix-Loop-Helix Homodimers
NO Helix-Loop-Helix Homodimers
Fig. 16.16
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Sex lethal regulation of transformer RNA splicing
Sex Lethal Protein
No Sex Lethal Protein
Stop codon
mRNA include a premature stop codon transformer
protein nonfunctional
after Fig. 16.16
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Regulation of Gene Expression via RNA Stability
RNA in situ
Northern Blot
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Mechanisms to Degrade mRNA

• Poly-A Tail Length
• Long tails confer stability
• Tail shortening leads to mRNA turnover
• Degradation Sequences
• Map to 3 (or 5) Untranslated Regions
• Interact with protein factors that degrade the
  targeted mRNA

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Reporter Constructs Are Used to Identify
Degradation Sequences
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