Chapter 17: Retroviruses

Question 1

All Retroviruses have the same gene order

Answer 1

5’- gag-pol-env-3’

Question 2

Non-coding regions

Answer 2

PBS
R
U5
U3
PPT

Question 3

PBS

Answer 3

Primer-binding site, complementary to 3’ end of tRNA

Question 4

R

Answer 4

Repeat sequence(150-200nt) at both ends (“teminally redundant”)

Question 5

U5

Answer 5

Unique sequence (240-1200nt) near 3’ end of genome

Question 6

PPT

Answer 6

polypurine tract(about 10 A/G), as a primer for (+)DNA synthesis during reverse transcription

Question 7

Attachment

Answer 7

Anti-receptor: SU protein
Receptor: depending on specific virus
conformational change in TM protein-> membrane fusion

Question 8

Entry and uncoating

Answer 8

Membrane fusion

Lose some protein-> from reverse transcription complex

Question 9

Reverse transcription

Answer 9

By RT: (+)RNA->(-)DNA->ds DNA
Occur in revers transcription complex(viral core)
tRNA
PPT
RNase H
Strand Transfer
Proviral DNA

Question 10

tRNA and PPT

Answer 10

as a primer for (-) DNA synthesis

Question 11

RNase H

Answer 11

digests RNA in RNA-DNA duplex(hybrid)

Question 12

Strand Transfer

Answer 12

During synthesis of 2 strand of 2 DNA strands, each detaches from its template and re-attaches at other end of the template via base pairing

Question 13

Proviral DNA

Answer 13

dsDNA made from revers transcription and is longer than RNA genome
One terminus acquired at U3 sequences
Other terminus acquired at U5 sequence
Both termini have U3-R-U5(Long Terminal Repeat)

Question 14

Integration

Answer 14

Pre-integration complex
Integrase
Provirus

Question 15

Pre-integration complex

Answer 15

Provial DNA associated with some viral proteins that enters the nucleus usually during mitosis

Question 16

Integrase

Answer 16

Cuts cell DNA and seals provirus gap

Question 17

Provirus

Answer 17

Integrated viral DNA-> “a part” of host chromosome; expressed immediately or pass to daughter cell

Question 18

Transcription and genome replication

Answer 18

• Two LTRs have identical sequence but different functions
• Upstream LTR: Promoter in U3; transcription starts at U3-R junction
• Downstream LTR: poly A signal; transcription stops at R-U5 junction
• Each transcript has 5’ cap

Question 19

3 Major genes translated into Polyproteins and cleaved into mature proteins

Answer 19

Gag and Gog-Pol proteins; Env proteins

Question 20

Gag and Gag-Pol proteins

Answer 20

Translated from full-lengths mRNA, on free ribosome
Myristylated at N-termini
2 mechanisms to translate gag-pol

Question 21

Reading through a stop codon

Answer 21

glutamine-tRNA (“suppressor tRNA”) mis-reads UAG(stop codon) as CAG

Question 22

Ribosomal frameshift (-1 shift)

Answer 22

change reading frame before gag stop codon

used by HIV-1

Question 23

Retroviruses nee much more Gag than Pol

Answer 23

i. About 95% of ribosomes terminate translation at gag stop codon
ii. About 5% of ribosomes continue translation to make Gag-Pol

Question 24

Env Proteins

Answer 24

Translated from spliced mRNA, in rough ER
Glycosylated
CLeaved by host protease into SU and TM in Golgi complex
Su and TM(associated)->further glycosylated->cell membrane

Question 25

Assembly

Answer 25

Usually at cell membrane
NC domain of Gag and Gag-Pol bind packaging signal near 5’ end of genome
tRNA binds to PBS
Many copies of Gag and a few copies of Gag-Pol-> coat RNA

Question 26

Exit

Answer 26

Myristyl groups of MA domain bind TM tails in cell membrane

Bud off with envelope

Question 27

Maturation

Answer 27

During and/or after budding
Viral protease Gag and Gag-Pol
o Gag structural proteins (MA, CA, NC)
o Gag-Pol  structural proteins and enzymes (PR, RT, IN)

Question 28

Retroviruses as Gene vectors

Answer 28

Recombinant REtroviruses

Question 29

Recombinant Retroviruses

Answer 29

genetically modified
carry foreign genes
Express the foreing genes at high level after integrated into cell genome