Lecture 7 - DNA Viruses: Papova 1 Flashcards
Eukaryotic mRNA is mono or polycistronic?
Monocistronic
Define monocistronic and polycistronic mRNA
Monocistronic mRNA: single open reading frame for one protein
Polycistronic mRNA: multiple open reading frames for multiple proteins
What does it mean for different ORFs to be in the same frame?
They have different start codons but the same stop codon
What 2 virus families does papovaviridae comprise?
Papillomaviridae
Polyomaviridae
Describe the morphology and genome of Papovaviridae
Nonenveloped, covalently closed circular dsDNA viruses
Describe the capsid structure of SV40 (3)
Structural unit: VP1 pentamer
Capsid: icosahedron of 72 VP1 pentamers
What is the receptor for SV40 and what does binding initiate?
Receptor is a glycosphingolipid, binding to it activates cellular tyrosine kinases and triggers actin filament reoganization and internalization
Describe the steps in SV40 entry (APU) (4)
1) Ganglioside receptor binding triggers internalization by caveolin-dependent endocytosis
2) Caveosome brings virions to the ER via a sorting vesicle
3) virions are released from the ER into the cytoplasm by an unknown mechanism
4) Capsid docks the nuclear pore complex and disrupts nuclear envelope and lamina organization
SV40 has a small genome, how does it produce large amounts of different proteins at different times and in different quantities? List 4 strategies they employ and the stage of MMS it occurs (transcr. etc.)
Transcription: differential regulation of promoters
Post transcription: alternative splicing (poly-adenylation)
Translation: leaky scanning
Post-translation: micro RNA regulation
What is the mechanism of action of SV40s enhancer region to promote transcription?
Enhancer promotes a nucleosome-free region allowing RNA polymerase binding
SV40 gene expression can be divided into 2 phases. What are they and when do they occur?
Early phase: before vDNA replication
Late phase: after vDNA replication
How many promoters does SV40 genome have?
2, early promoter and later promoter
What is the SV40 early gene product? What does it promote and inhibit?
Large T antigen:
Promotes: S phase induction, late gene transcription and vDNA replication
Inhibits: early gene transcription
What type of proteins are the SV40 late gene products?
Structural proteins
What is the regulatory region of SV40 genome called?
Ori
What are the 2 promoters located in the SV40 Ori? What do they make?
Early promoter: T antigen
Late promoter: structural proteins
What is the directionality of the 2 promoter elements in Ori?
Early and late promoters are transcribed in opposite directions
The first step in SV40 gene expression involves production of ___ antigen by basal ___ factors.
First step in SV40 gene expression involves production of T antigen by basal transcription factors
The ___ region of SV40 genome facilitates transcription initiation of the early genes encoding ___ by preventing ___ formation, thereby stimulating RNA polymerase binding
The enhancer region of SV40 facilitates transcription initiation of the early genes encoding T antigen by preventing nucleosome formation, thereby stiulating RNA polymerase binding
The second step in SV40 gene expression is early gene mRNA splicing to make ___ and ___
SV40 early gene mRNA is spliced to make large T antigen and small T antigen
In the third step of SV40 gene expression, TAg interacts with host factors ___ and ___ to induce ___ phase
TAg interacts with host factors Rb and p53 to induce S phase
In the fourth step of SV40 gene expression, hexamers of what bind to the Ori?
TAg hexamers
What are the 3 functions of large T antigen when bound to the Ori of SV40?
- increase vDNA synthesis
- increase late gene expression
- decrease early gene transcription
TAg is an early gene product. When it binds to the Ori it decreases early gene expression. What form of regulation is this?
Autoregulation
How does TAg directly increase late gene expression?
TAg hexamerizes at the Ori and stabilizes transcriptional complexes for late genes
In the fifth step of SV40 gene expression, increased ___ concentration titers out a repressor of ___ promoter, allowing for expression of ___ genes
Increased vDNA concentration titers out a repressor of the late promoter, allowing for expression of late genes
How is repression of the late promoter for SV40 removed?
High viral DNA concentration titrates out the repressor of the late promoter (more promoters than there are repressors)
What are 2 reasons that alternative splicing of viral pre-mRNA is important for viruses?
- expands the coding capacity of their genome
- deals with the monocistronic constraint of euaryotic translation machinery
What are 2 ways in which LTAg assists in replication of SV40 vDNA?
- acts as helicase to unwind DNA
- binds Topoisomerase 1 to relieve positive supercoils
