Site-specific DNA condensation and pairing mediated by the int protein of bacteriophage lambda

The int protein of bacteriophage lambda catalyzes the site-specific integrative recombination that inserts lambda DNA into the host chromosome. The attachment site region of lambda DNA required for this reaction spans 230 base pairs and includes four separable binding sites for int protein. We have used the electron microscope to determine the functional consequences of the interaction of int with its multiple binding sites. We find that int condenses a 230-base pair segment of DNA into a compact structure about 14 nm in diameter; the condensed region includes all of the four binding sites for int. Condensed segments will form paired structures between attachments sites. We suggest that a sequential cooperative interaction between bound int molecules provides for a distinct reactive DNA conformation and for pairing between substrate sites.