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PLANT VIROLOGY - THIRD EXAM - 1987

Answer questions directly with minimum words.

1- If a virus takes 2 hours to sediment at 50,000 RPM, how long will it take to sediment at 20,000 RPM. (s = kwr²). Show calculations.

2- Isolated plant virus capsid proteins are generally much easier to denature than proteins that are part of the virus capsid. Why? Answer in less than 25 words.

3- Related viruses one and two contain antigenic sites ABC, and ABD respectively. What will the immunodiffusion pattern look like if the viruses are compared to one another with antiserum to virus one? What will the immunodiffusion pattern look like if the experiment is repeated, but the antiserum is incubated with an excess of virus two before it is placed in the immunodiffusion well? What will the immunodiffusion pattern look like if the original experiment is repeated, but the two virus samples are incubated with an excess of antiserum to virus two before being placed into the immunodiffusion wells?

4- RNA is isolated from an unknown plant virus. When it is translated in vitro it produces 3 proteins of 110 kD, 150 kD and 35 kD. None of the proteins reacts with antiserum to the capsid protein. What would you guess to be the functions of the three proteins? By gel electrophoresis the purified RNA appears to be a single component. How could you explain that it appears to be translated into three different peptides? How is the viral coat protein likely to be translated? Answer all three questions in less than 30 words.

5- Two strains of a DNA virus are partially sequenced. The nucleotide sequences differ by 2%. A restriction endonuclease which recognizes four base pairs cleaves the DNA of one of these strains 6 times. What is the probability that it cleaves the other strain 6 times? (i.e. the probability that mutation does not change the restriction pattern)

6- You translate a virus in vitro to give 5 peptides. If you stop the translation after 15 minutes with the inhibitor cycloheximide, you discover that on further incubation the larger peptides gradually "disappear". Two possibilities should be considered, first that the translation product is a polyprotein and second that proteases in the translation system may nonspecifically degrade the proteins. By simply observing gels of in vitro products at various times before and after cycloheximide treatment how could you make at least a preliminary guess as to which of these mechanisms is at work? Answer in less than 30 words.

7- A spherical virus, with 25% RNA, has an ultraviolet absorption spectrum of about the same shape as that of tobacco mosaic virus, which has only 5% RNA. What does this tell you about the virus? Answer in less than 10 words.

8- Imagine that you have inserted a pseudogene into tobacco mosaic virus and that the product is infectious. What is likely to happen to the pseudogene as this strain of virus is continually cultured in the greenhouse.? Answer in less than 20 words.

9- For Maxam and Gilbert sequencing (chemical method), DNA is usually labelled at the 5' end with polynucleotide kinase. Much higher specific activities could be obtained by synthesizing DNA in vitro using a-32-P labelled deoxyribonucleoside triphosphates. Discuss the potential value of using in vitro synthesis to enhance Maxam and Gilbert sequencing. Answer in less than 40 words.

10- Antibody is prepared to a synthetic peptide corresponding to a sequence within the capsid protein gene of a plant virus. Is this antibody more likely to react with the capsid protein on Western blots or to react with the intact virus particle? Why? Answer in less than 20 words.