Chapter 6, Problem 39P

ΦC31 is a type of bacteriophage that infects Streptomyces bacteria. One gene in the bacteriophage genome specifies a recombinase called ΦC31 integrase that works through a mechanism slightly different from that of the recombinase shown in Fig. 6.30. Most importantly, the two target DNA sequences are different from each other. One called attP is 39 base pairs and is found on the circular bacteriophage chromosome, while the other—attB—is 34 base pairs long and is located on the much larger circular bacterial chromosome. Excepting two base pairs roughly in the middle of both targets that are identical and at which recombination takes place, the DNA sequences of attP and attB are completely different from each other.

a.	Diagram the reaction that ΦC31 integrase performs. How could this reaction be important for the life cycle of the bacteriophage?
b.	Using the diagram you just drew, explain why ΦC31 integrase cannot reverse the reaction.
c.	Now consider how you might exploit this site-specific recombination to place genes from another species (a transgene) into the genome of an experimental organism like Drosophila. Assume you can make any DNA sequences you want and that you can introduce these DNA sequences into fruit fly germ-line cells by injection. Why is the irreversibility of the ΦC31 integrase–mediated reaction valuable for placing the transgene into the Drosophila genome?
d.	Bacteriophage ΦC31 must eventually reverse this reaction. Why? How do you think the bacteriophage can achieve this reversal?