Binding and bending of the lambda replication origin by the phage O protein.

We have characterized the binding of lambda phage replication initiation protein O to the phage origin of replication. The minimal DNA segment required for O binding is the single iteron, a 19‐bp sequence of hyphenated dyad symmetry that is repeated with variations four times in the origin. The isolated amino terminus of O protein is also sufficient to bind DNA. Electrophoretic studies show that the amino terminus of O protein induces bending of a single iteron. The DNA‐protein interaction was characterized by ethylation interference, dimethyl sulfate protection and neocarzinostatin footprinting. Points of DNA‐protein contact are largely concentrated in two areas symmetrically disposed with respect to the dyad symmetry of the iteron. This suggests the protein interacts as a dimer with half sites in the DNA. However, a few non‐symmetrical contacts are found, indicating that O protein may distort the helix. This may correlate with the bending effects demonstrated electrophoretically. Cylindrical DNA projections were used to model O protein binding to the lambda origin and compare it with the lambda repressor‐operator interaction. Whereas bound repressor nearly encircles the DNA in the major groove, O protein leaves the major groove on the opposite side exposed.