Transcript Powerpoint Slides 6.1 Part A

Unit 6.1 2015
Gene Expression and
Replication in Small DNA
Viruses
Microviridae
Small DNA Viruses of Animals
(Papovaviruses)
What do the small DNA viruses have to
tell us about viral approaches to gene
expression and replication?
What are their issues, problems and
strategies?
Are there common themes?
Microviridae-PhiX174
Historically important virus because…
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first viral genome to be sequenced (1977)
+ sense ss DNA
circular
5386 nucleotides-very small genome
genome infectious by itself
model for fundamentals of DNA replication
PhiX174 –among smallest DNA
viruses
T=1 Capsid
Ss + DNA circle
Converted to ds immediately upon entry = “RF”
11 gene products identified
Attachment and Penetration of PhiX174
Gram Negative Cell Wall
•Attaches via vertex & pilot protein H binds to a
specific lipopolysaccharide in outer cell membrane of
its host, E. coli or S. typhimurium
•The pilot protein leads the circular genome into the
cell by an energy-dependent mechanism
•The genome attaches to the inner bacterial
membrane in preparation for DNA replication and RF
formation.
11 Gene Products
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A: DNA replication (513)
 A*: inhibit host replication????? (341)
 B: capsid assembly protein (internal) (120)
 C: regulation of replication (86)
 D: capsid assembly protein (external) (152)
 E: lysis (91)
 F: major capsid protein (427)
 G: spike protein (175)
 H: pilot protein (328)
 J: DNA binding protein(capsid assembly) (38)
 K: Burst size regulation???? (56)
Genome is very small for 11 gene
products
Genome sequencing
reveals answer
Seven genes overlap at
least one other gene
Gene for A* overlaps 3
other genes
Frameshift or use of
alternate AUG codon
Non-capsid genes
involved
Rolling Circle Replication-”sigma”
The RF is made by
host enzymes
(-) strand template
for (+)
Multiple copies of
(+) made in a chain
Single genomes cut
out
(+) is nicked by A
protein
Small genome = Dependence on
Host
RF is produced by action of host
DdDp enzymes
Progeny genomes synthesized by
rolling circle mechanism (host and
viral protein A)
Transcription by host DdRp
enzyme