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Transcription from DNA Virus Genomes
Lecture 8Virology W3310/4310
Spring 2012
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Viruses are Informative
• Control signals
• Nature of a promoter
• What an enhancer is
• What introns and exons are
• How RNA synthesis is initiated and regulated
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Paradigms for Transcription
• One of the first events following infection
• Variety of “chromosome - like” templates -Polyomaviridae - a regular array of nucleosomes -Adenovirus and Herpesvirus - chromatin-like DNA structures -HIV - transcribed from integrated DNA
• Regulation, expression is strictly defined
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Transcription
• Regulation is primarily controlled by initiation - instances where elongation is a rate controlling step
• A multi-step process, other opportunities for control
• Specificity of initiation
• Termination - both polymerase and RNA are released from template
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Generic Steps in Transcription
• Steps are just like DNA replication
• Promoter recognition
• Preinitiation complex formation
• Initiation -site specificity
• Elongation
• Termination
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What Happens to RNA Transcripts?
• Capping
• Polyadenylation
• Splicing
• Editing
• Transport -becomes mRNA, get translated
• Decay, t1/2 is critical
• Silencing -degradation -inhibition of translation
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Host Polymerases
• Pol I - pre rRNA not known to be used by viruses
• Pol II - makes mRNAs and some micro RNAs
• Pol III - Adenovirus VA RNAs, EBV EBERs and some micro RNAs
• How does the virus subjugate the host?
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Prerequisites for Transcription
• Hepatitis B Virus - fill it first!
• Adenovirus, Polyomaviruses - enter cell nucleus
• Herpesviruses -introduce virion-associated proteins
• Retroviruses - +RNA - dsDNA - Integrate
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Transcriptional Programming
• Regulation of synthesis
• How? -control timing and abundance
• Why? - orderly synthesis allows for specific events -some gene products might be toxic
• What happens if things go awry?9
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Steps in Transcription of pre-mRNA
Many initiation events abort
At termination both Pol andtranscript are released
Elongation is frequently terminated
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Promoter Control Elements
• Core and distal elements, specific DNA sequences
• TATA - defined sequence - TFIID
• Initiator - specify accurate starts
• Distal - sites for upstream (or downstream) activator proteins
• Enhancers - position and orientation independent DNA elements- tissue specific or universal
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Promoter Structure
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Templates
• Enter the nucleus
• Templates and accessory proteins - early gene expression
• Produce a recognizable template for transcription of first wave of virus genes
• Replicate genomes to increase template #- consequences
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What Does Pol II Do?
• A large complex assembly - holoenzyme
• Recognize the promoter
• Specify accurate initiation
• Responds to host and virus proteins
• Synthesize RNA transcripts
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Steps in Initiation
• Recognition of Core Promoter
• Formation of stable closed initiation complex
• Formation of open initiation complex
• Escape from promoter -regulation of Pol II by Phosphorylation -promoter clearance - elongation -movement of Pol complex
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Order of Binding
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i) TfIId -Tbp & Tafs- bind & bend DNAii) TfIIa enters facilitating binding of Tbp to DNAiii) Formation of closed initiation complex
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Order of Binding
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Changes to C’ of Pol leads to promoter clearance & elongation
Recognition - stepwise assembly TBP & TAF’s bend DNA
Unwinding - Pol remains in contact with promoter
Release from promoter
Initiation
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Assays for Promoter Activity
• Run off assays - in vitro
• Pulse labeling - in vivo
• Reporter assays - in vivo
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Ways to Assay Transcription
In vitro Transcription vs. Reporter Assays21
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Further Steps in Regulating Transcription
• Regulation of abundance through initiation
• Availability
• Decoration of co-activators, P, Me, Ac
• Role of enhancers - change rate of initiation
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Splicing
• Nuclear RNAs (hnRNAs) > mRNA
• hnRNAs have 5’ caps and 3’ poly A
• All Adenoviral L RNAs map to the same promoter
• Adeno L mRNAs have 4 parts, 5’ terminal tripartite leader and body
• How to get small RNAs from big RNAs?
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Adenovirus Transcription Map
• All transcription dependent on E1A• Late transcripts have common 5’ end• Eight transcription units, unique mRNAs
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MLP-leader Sequence
DNA loops
Why does the DNA loop out?
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Splicing = Value Added
• Introns provide numerous sites at which RNA sequences are broken and rejoined
• Splicing occurs without loss of coding information = economical
• Alternative splicing creates new functional genes
• Coding information of a small DNA genome is expanded
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Adenovirus Alternative Splicing
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HIV Alternative Splicing
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Enhancement of Sequence Dependent Initiation, the Polyomaviridae Enhancer
• The enhancer is a duplicated 72bp sequence
• Works at a distance
• Orientation independent
• Can work in trans
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Enhancer Structure
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How do Enhancers Work?
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Enhancers Work in Trans
Digest
Digest + biotin
Intact
Link
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Regulation of and by Host Proteins
• Viruses use host and/or virus-specified proteins to regulate gene expression
• They either encode and/or bring with them co-activating molecules
• Cell type specificity can limit expression - co-activator molecules can be organ or species specific
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Regulatory Protein Domains
• Regulatory molecules are composed of multiple domains that contribute to virus gene regulation
• DNA binding
• Activator/Repressor
• Interactor
• Multimerization
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Viral Transcriptional Activators
• Autoregulatory molecules -SV40 Tag, HSV ICP4
• Some bind DNA - T, EBNA, ICP4, E2
• Some bind host proteins - HSV VP16
• Some liberate host TA’s - T, E1A, E7
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Patterns of Regulation• Proteins interact with Pol II to establish regulatory
circuits
• Positive Autoregulatory Loops -alter the rate of transcription initiation -virus proteins stimulate transcription
• Negative Autoregulatory Loops -repress gene expression
• Transcriptional Cascade -transcriptional units are activated in a fixed sequence 36
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Regulatory Machines
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Positive vs. NegativeAutoregulatory Loops
Cascade Regulation
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Transcriptional Cascade• Allows for transcription of viral genes in a
reproducible and temporally controlled sequence
• Immediate early and early proteins
• Transcription of late genes
• Ensures coordinated production of DNA genomes and structural proteins, frees template from repressors
• Activating proteins can induce transcription of host and viral genes and repress transcription of their own genes
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Transcription Made Easy
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DNA Virus Transcription
• Pollyomavirus 5kbp, 2 units, 6 proteins -heavy reliance on host to get started -splicing generates multiple RNAs
• Adenovirus 35kbp, 8 units, 40 proteins -minimal # of transcription regulatory sequences
• Herpesviruses 150kbp, ~80 units, more proteins
• Virus-specified transcriptional activators -some autoregulate - T, ICP4 -others just activate - E1A
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Polyomavirus Transcription
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• E and L units transcribed from a common region, no nucleosomes
• E and L transcripts contain overlapping mRNAs, regulated by splicing, share poly A sites
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How Does T Work?
• T binds polyomaviridae Oris as a hexamer
• Early promoter dampened
• Late promoter activated
• Early transcripts are decreased relative to Late
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Adenovirus Transcriptional Regulation
• Three virus proteins and DNA synthesis govern phase transitions
• E1A, necessary for transcription of all E transcription units
• E2 required for DNA synthesis and entry into L transcription phase -increases initiation from MLP
• IVa2 enhances L gene transcription
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Adenovirus Transcription Units
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E1A Gene Transcript Family
• Differential splicing results in two proteins of 243 and 289 amino acids with a conserved reading frame
• CR3 stimulates early gene transcription
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SD
SD
SA
SA
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How Does E1A Work?
• E1A does not bind DNA
• E1A does bind , Atf-2, Sp1 and Med23 -binding to Med23 stimulates assembly of preinitiation complexes
• Also activates by another mechanism -interaction with host regulatory proteins
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Interaction of E1A with Rb
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Herpesvirus Regulatory Cascade
• Initiated by VP16, a virion associated protein
• Activates IE transcription
• IE proteins control transcription from all virus genes
• Expression of E genes and DNA synthesis
• Expression of DL and L gene, DNA dependency
• Packaging of VP16 into new virions
• Coordinate regulation in a temporal fashion
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Now for Something Simple
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Distinct Patterns of Accumulation of HSV RNA
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VP16
• Potent C-terminal acidic activator
• Does not bind DNA directly -but requires a TAATGARAT motif in virus promoters
• Associates with HCF and Oct-1 -they provide promoter specificity
• Stimulates initiation and elongation of transcription
• Specific for IE promoters
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Interactions by VP16
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Export
• A primary transcript does not become a mRNA until it is exported
• Export is usually accomplished by host proteins and the transcript uses nuclear pores to exit
• A protein complex that marks mature RNAs for export from the nucleus is assembled during splicing
• Exportins shuttle between the nucleus and cytoplasm carrying RNA as their cargo
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Today’s Concepts
• Transcription is complicated
• Control is at many levels
• Host and viral proteins regulate transcription
• Viral gene expression is coordinately regulated in a temporal manner
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