Plasmid and catenane substrates for EcoP15I and EcoPI. Recognition sites for either EcoP15I (a) or EcoPI (b) are indicated by black arrows. The locations of the Type III site are defined according to (4). Recombination sites for Tn21 resolvase (21res) are indicated by grey arrows. Daughter catenanes are illustrated directly below their corresponding parental plasmid: i.e. recombination of pMDS34a generates 34acat, etc. The numbers in parentheses correspond to total size of the plasmid, or, for the catenanes, to the sizes of the large and small rings, respectively.