The incA locus of plasmid P1 controls plasmid copy number by inhibiting the replication origin, oriR. Both loci contain repeat sequences (iterons) that bind the P1 RepA protein. Regulation appears to occur by contact of incA and oriR loci of daughter plasmids mediated by RepA-bound iterons. Synthetic incA iteron arrays were constructed with altered numbers, sequences or spacing of iterons. Using these in in vitro and in vivo assays, we examined two models: (i) that the origin and incA loci form a stable 1:1 complex in which multiple iterons of each locus are paired with those of the other, and (ii) that individual incA iterons act as freely diffusing nucleoprotein units that contact origin iterons in a random and dynamic fashion. The data presented here strongly favour the latter case. The origin, with its five iterons, acts as a target but not as an effector of regulation. We present a model for replication control based on random, dynamic contacts between incA iterons and the origin. This system would display randomness with respect to choice of templates and timing of initiation if multiple replicon copies were present, but would tend to act in a machine-like fashion in concert with the cell cycle if just two copies were present in a dividing cell.