Problems

1 The table below shows the properties of the genomes of three different viruses. The data were obtained as follows: Nuclease sensitivity was measured by the ability of deoxy-ribonuclease (DNase) or ribonuclease (RNase) to destroy the genome (a "+" means sensitivity). The ability of the genome to act as mRNA was tested by incubating it in a cell-free system. If amino acids were incorporated into protein, the data are shown as a "+." Finally, the virus particles were tested for the presence of a virion polymerase. If an enzyme was present, the data show whether it could polymerize deoxynucleotide triphosphates (dNTPs) or nucleoside triphosphates (NTPs).

Genome properties

Can genome be an

Nuclease sensitivity?

mRNA?

Virion polymerase?

Virus

DNase RNase

with dNTPs

with NTPs

#1

- +

+

-

-

#2

- +

-

-

+

#3

- +

+

For each virus, indicate the strategy of the genome, using the Baltimore classification. What is the nature of the product of the virion polymerase when present?

2 Interferons are synthesized by cells in response to many different viral infections. The common result of the interferon-induced antiviral state is the cessation of protein synthesis. Predict the effect of the following treatments of the indicated cell on protein synthesis in that cell. (Assume, for the purpose of this question, that the virus does not inhibit cellular protein synthesis as a result of the infection.)

Viral infection of cell

Insertion of dsRNA into cell

Normal cell

Interferon-treated cell

3 You wish to produce a subunit vaccine for a positive-sense RNA virus that will stimulate the production of neutralizing antibodies in the person receiving it. Indicate which of the following viral proteins would be a logical candidate for such a subunit vaccine and state a brief justification.

  • a) Viral capsomer protein.
  • b) Viral protein that is bound to the RNA genome inside of the virion.
  • c) Viral RNA polymerase.

4 Each year in late winter a behavioral "disorder" engulfs the people of New Orleans, Louisiana, reaching a climax on the day before Ash Wednesday. Together with virologists at Louisiana State University, you have isolated a virus from the affected people that you suspect is responsible for this condition. You have named the new isolate Mardi Gras virus (MGV). You have found a convenient host cell in which to grow MGV in the laboratory. The following table lists some of the properties of MGV you have discovered.

Initial data for Mardi Gras virus

Experiment

Observation

Physical nature of the virion

Electron microscopy (EM) reveals 100-nm particles; shape indicates presence of envelope with visible surface projections; ether destroys particle integrity

Chemical nature of viral genome

Digested with RNase; degraded by alkali; resistant to DNase

Informational nature of viral genome

Genome cannot be translated in cell-free protein synthetic system

Enzymatic analysis of virion

With NTP precursors: catalyzes RNA synthesis; with dNTP precursors: no reaction

Biological analysis of virus

HeLa cells (human): attachment and penetration (observed by EM) and progeny virus produced; AGMK cells (simian): attachment and penetration (observed by EM) but no progeny virus produced

  • a) What would you predict to be the effect of treatment with ether or other lipid solvents on the infectivity of MGV?
  • b) To which Baltimore class would you assign MGV? Give two reasons for this classification, based on the data in the table.
  • c) Based on the data in the table, would you say that MGV is a human or a simian virus? Justify your answer briefly with reference to the data.

5 If a virus has a negative-sense RNA genome, what enzymatic activity (if any) will be found as part of the virion structure and what will be the first step in expression of the viral genome?

6 Influenza viruses gain entry into their host cells by attachment to N-acetylneuraminic acid residues on the cell surface, followed by receptor-mediated endocytosis. Predict what effect the treatments shown in the table below will have on (a) the attachment of an influenza virus to a susceptible host cell, and (b) the subsequent uncoating of influenza virus in the same cell. Use a "+" to indicate that the event will take place or a "—" to indicate that it will not take place. Note: In each case it is assumed that the events would be occurring in the same cell that has undergone the treatment.

Treatment

Attachment?

Uncoating?

Treatment of the host cell with neuraminidase

Treatment of the host cell with NH4Cl, which prevents lowering of the lysosomal pH

Treatment of the host cell with actinomycin D, which prevents synthesis of messenger mRNA

7 Cells produce mRNA by transcription of their DNA genomes. By contrast, single-stranded RNA genome viruses have three different strategies with respect to viral mRNA production. Briefly describe the production of mRNA for each of the following viruses.

  • a) Poliovirus.
  • b) Vesicular stomatitis virus.
  • c) Rous sarcoma virus.

8 Infection of a human with influenza virus can trigger both host defense systems: the interferon response and the immune response. In the table below, indicate with a "Yes" or a "No" which of the events is characteristic of which defense system (either, both, or neither).

Event

The interferon response

The immune response

Both host and viral mRNA are degraded in the cell after infection

A fragment of viral protein in complex with class I MHC is displayed on the surface of the infected cell

The virally infected cell dies

Capped mRNA is no longer translated in the infected cell

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