Helper packaging strategy.

The large and small rectangles represent the envelope and chromosome of an E. coli cell with three prophages. Prophages1 and 2 are inserted in tandem in the bacterial chromosome, and the circle represents a third, heat-inducible plasmid prophage, the helper prophage. Induction of the helper prophage to lytic growth provides packaging components for both helper DNA and prophage DNA, provided the helper’s terminase can initiate packaging at the cos of upstream prophage 1 and terminate packaging at the cos of downstream prophage 2. The DNA packaged from the tandem prophages is represented by the horizontal arrow. The tandem prophages are not heat-inducible and remain repressed during the experiment.