Lambda excision revisited: testing a model for synapsis of prophage ends.

Excision of lambda prophage was reexamined to test a model for prophage end synapsis. The model proposes that, during in situ prophage replication, following induction, the diverging replication forks are held together. Consequently, prophage DNA is spooled through the replication machinery, drawing the prophage ends together and facilitating synapsis. The model predicts that excision will be slowed if in situ lambda replication is inhibited, and the predicted low rate of excision of a nonreplicating prophage was observed after thermoinduction. However, excision was rapid if additional Int protein was supplied or if the temperature was reduced after induction, showing that (i) Int is partially thermosensitive for excision at 42 degrees C and (ii) in situ replication is not required for rapid excision, a finding that is inconsistent with the model.