Fig. 4.

Construction of a non-att Holliday structure. The HindIII fragment of λ DNA ( ), which extends from 57.0% to 52.4% on the λ map and does not contain an att site, was cloned as a tandem duplication into the HindIII site of pBR322 (—), to generate plasmid pPH910. Three HinfI fragments of pPH910 were isolated for the χ-form construction: fragment II consists of the left pBR322-λ junction, fragment III of the head-to-tail junction of the two λ fragments, and fragment IV of the right λ-pBR322 junction. Fragment I is the pBR322 HinfI fragment into which the λ DNA had been inserted. For each fragment, and for each arm of the χ-structure, the amount (base pairs) of pBR322 DNA or λ DNA is indicated.

Methods: The χ-form was generated by alkali denaturation and renaturation according to scheme (2) and purified by agarose gel electrophoresis as described in Fig. 2 legend.