FIGURE 1.

Schematic representation of the R6K replicon and models corresponding to origin activation. A, replicon of R6K showing the approximate locations of the origins α, β, and γ; note that ori γ contains 7 iterons, ori β a half-iteron, and ori α a single iteron, respectively. The operator of π ORF contains the 8th iteron and two half-iterons present as inverted repeats. There are 2 dnaA boxes at ori γ. B, models showing looped structures, which are mutually exclusive and correspond to the turning on of α, β, or γ. C, postulated structure of a pair of handcuffed γ origins that leads to turning off of both. In the active origins, the iteron DNA is believed to be wrapped around a core of monomeric π that is postulated to be unraveled in the handcuffed ori γ. C represents negative control of replication in trans, whereas the models in B show origin activation in cis.