Retroviruses

Question 1

RNA viruses that replicate through a DNA intermediate, which efficiently integrates into the genome of permissive cells forming a provirus

Answer 1

Retroviruses

Question 2

A hallmark of the retrovirus life cycle is the process of

Answer 2

Reverse transcription

Question 3

The viral RNA genome acts as template for the synthesis of viral DNA by the virus encoded polymerase during

Answer 3

Reverse transcription

Question 4

Other viruses such as Hepatitis B virus and Cauliflower Mosaic virus utilize

Answer 4

Reverse transcription

Question 5

The study of Retroviruses has taken on added intensity since the discovery of pathogenic human retroviruses such as

Answer 5

HIV-1 and 2 and Human T-lymphotrophic virus (HTLV-1)

Question 6

Cause tumors in animals

Answer 6

Oncoretroviruses

Question 7

Cause a variety of immunological and neurological diseases with a long latency

Answer 7

Lentiviruses

Question 8

Cause vacuolation of some mammalian cells in culture

Answer 8

Spumaviruses

Question 9

Surrounded by a lipid envelope derived from the infected cell upon budding of the virus

-Measure approximately 100 nm in diameter

Answer 9

Mature HIV-1 Virion

Question 10

The virions have spikes emanating from the

Answer 10

Envelope

Question 11

The spikes are composed of virus envelope proteins, which bind to receptors on permissive cells to initiate

Answer 11

Infection

Question 12

The core of mature virions is conical in structure in the case of

Answer 12

HIV-1

Question 13

The core of mature virions contains a round core in the center of the particle in the case of the

Answer 13

Murine leukemia virus

Question 14

There are additional structures that have been observed for other types of retroviruses. But what is common to them all is that the infectious mature virions contain a condensed core of

Answer 14

Electron dense material

Question 15

The genome within the core consists of two usually identical single-stranded, positive strand

Answer 15

RNA molecules

Question 16

The RNA is capped and polyadenylated indicating that it is transcribed by cellular

Answer 16

RNA polymerase II

Question 17

Most of the cis-acting elements that must be covalently linked to the genome for efficient replication to occur are located at the ends of the

Answer 17

Genome

Question 18

The trans-acting sequences encoding the viral proteins are located between the

Answer 18

cis-acting elements

Question 19

The retroviral genome is always organized in the same way as

Answer 19

5’ gag pol env 3”

Question 20

At each end of the proviral genome (integrated double-stranded DNA) are

Answer 20

Long Terminal Repeats (LTRs)

Question 21

The long terminal repeats can be funcitonally divided into which 3 parts?

Answer 21

U3, R, and U5

Question 22

Begins in the 5’ LTR of the provirus at the U3-R border

Answer 22

Transcription initiation

Question 23

It should be noted that the viral promoter is harbored in

Answer 23

U3

Question 24

Occurs in the 3’ LTR at the R-U5 border

Answer 24

Polyadenylation

Question 25

Thus, the single-stranded RNA genome found in virions is shorter than the viral doublestranded

Answer 25

DNA genome

Question 26

The sequence that is lost during RNA transcription is replaced during

Answer 26

Reverse transcription

Question 27

They are typically cell surface proteins with the different groups employing different proteins, as such they are important determinants of host range

Answer 27

Retrovirus receptors

Question 28

A notable retrovirus receptor -The receptor for HIV

Answer 28

CD4

Question 29

Found on helper T cells and macrophages helping to account for the HIV host range

Answer 29

CD4 receptor

Question 30

In contrast to other retrovirus receptors isolated to date, requires a co-receptor

Answer 30

HIV infection

Question 31

The co-receptors are members of the chemokine receptor family with the most prominent being

Answer 31

CXCR4 and CCR5

Question 32

Infection is initiated when the envelope glycoprotein SU (surface envelope protein) binds to the corresponding receptor(s) on the permissive

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Target Cell

Question 33

This initiates conformational changes resulting in direct membrane fusion between the membranes of the

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Virus and cell

Question 34

This fusion is mediated by the hydrophobic domain of

Answer 34

TM

Question 35

After entry into the cell, the process of reverse transcription ensues during which the single-stranded virion RNA is converted into double-stranded DNA by the virus encoded

Answer 35

Reverse transcriptase

Question 36

In order to generate the LTRs, RT makes two -Unusual for a DNA polymerase

Answer 36

Strand transfers

Question 37

Can integrate into non-dividing cells indicating that there is an active transport mechanism for the preintegration complex

Answer 37

HIV-1

Question 38

On the other hand, other retroviruses integrate during

Answer 38

Mitosis

Question 39

Integration occurs relatively randomly throughout the genome with a preference for open chromatin such as

Answer 39

Actively transcribed genes

Question 40

Mediated by the virus encoded integrase (IN) and requires cis-acting attachment sites at the ends of the viral genome

Answer 40

Retroviral integration

Question 41

During the process of integration, two nucleotides are lost at each end of the unintegrated dsDNA genome and the cellular target sequence is

Answer 41

Duplicated

Question 42

The retroviral genome is transcribed by cellular

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RNA polymerase II

Question 43

The viral transcripts then undergo RNA processing and

Answer 43

Transport

Question 44

For all retroviruses at least two mRNAs are produced: 1) a full-length transcript which is translated to yield the

Answer 44

Gag and Gag-Pol polyproteins

Question 45

For all retroviruses at least two mRNAs are produced: 2) a singly spliced mRNA, which encodes the

Answer 45

Env polyprotein

Question 46

However, this represents the simplest splicing pattern as exemplified by

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Moloney murine leukemia virus

Question 47

Has a more complex splicing pattern because in addition to the expression of Gag, Pol, and Env, it encodes a number of other proteins translated from alternately processed transcripts

Answer 47

HIV-1

Question 48

The gag and pol genes are usually in different translational reading frames, yet both are translated from the same unspliced

Answer 48

Viral mRNA

Question 49

Most of the time just the Gag polyprotein is translated, but a percentage of the time (e.g. for HIV-1 about 5%) a larger Gag-Pol polyprotein is synthesized owing to

Answer 49

Translational frameshifting

Question 50

One notable exception to translational frameshifting is in the case of MLV in which there is a stop codon between the

Answer 50

Two genes in the same reading frame

Question 51

Again RNA secondary structure seems to play an important role in suppressing the stop codon in the case of

Answer 51

MLV

Question 52

Once translated the Gag and Gag-Pol polyproteins are transported through the cytosol to the inner cell membrane where assembly takes place for most

Answer 52

Retroviruses

Question 53

The polyprotein is proteolytically processed by the virus encoded

Answer 53

Protease

Question 54

Translated from a singly spliced viral mRNA

Answer 54

Envelope glycoproteins

Question 55

The Env polyprotein matures in the

Answer 55

Secretory pathway

Question 56

It has a signal peptide for cotranslational insertion into the endoplasmic reticulum (ER), and there is a hydrophobic domain, which anchors it to the membrane

Answer 56

Env polyprotein

Question 57

In the ER processing begins including significant glycosylation followed by transport to the

Answer 57

Golgi complex

Question 58

Here, a cellular protease further cleaves Env polyprotein into

Answer 58

SU and TM

Question 59

This distinguishes it from Gag and Gag-Pol processing, which is carried out by the

Answer 59

Viral protease

Question 60

Remain associated via noncovalent interactions

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SU and TM

Question 61

For most retroviruses, assembly and budding occur at the

Answer 61

Cell surface

Question 62

The Gag and Gag-Pol polyproteins have been targeted to the cell surface through the

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Cytosol

Question 63

Once all of the virus components are localized to the cell membrane, we see

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Assembly into virions

Question 64

Budding then ensues with subsequent viral maturation when the virus protease cleaves the

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Gag and Gag-pol polyproteins

Question 65

This produces the mature and infectious

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Virion

Question 66

Regulation of processing is controlled because protease is only active as a dimer, so it only becomes active

Answer 66

Post-assembly

Question 67

Most retroviral vectors used for gene therapy trial have been derived from the simple

Answer 67

Murine Leukemia Virus (MLV)

Question 68

A common strategy used for making the system safer is to separate the

Answer 68

Cis-acting and trans-acting sequences

Question 69

The gene(s) that you want to transfer is placed into a transducing vector, which contains the

Answer 69

Cis-acting sequences

Question 70

However, the transducing vector has deletion of some or all of the

Answer 70

Trans-acting sequences

Question 71

A packaging cell is needed in order to propagate the

Answer 71

Vector virus

Question 72

This cell expresses the genes encoding the viral protein using non-retroviral signals to direct their

Answer 72

Expression

Question 73

The transducing vector, in the form of plasmid DNA, is introduced into the packaging cell, where the vector virus RNA is transcribed and packaged into

Answer 73

Vector virions

Question 74

The vector virus buds from the packaging cell and can be used to target the cells of

Answer 74

Interest

Question 75

Since the vector virus does not encode the viral proteins, the vector can integrate into the genome of the target cell but can not produce

Answer 75

Progeny virus

Question 76

Recent trials that exemplify both the potential benefit and the risk of gene therapy are those involving the treatment of

Answer 76

Severe Combined Immunodeficiency (SCID)–X1

Question 77

SCIDs are a series of genetic disorders in which T-cell differentiation is arrested resulting in severely impaired

Answer 77

Immunity

Question 78

Ten different molecular defects have now been identified, all of which lead to early death in the absence of

Answer 78

Therapy

Question 79

Transplantation of MHC compatible hematopoietic stem cells can restore T-cell development and rescue the lives of

Answer 79

SCID patients

Question 80

An X-linked disease

Answer 80

SCID-X1

Question 81

Contains early progenitor cells that can repopulate the lymphoid lineage

Answer 81

CD34+ population