FIGURE 4.

Data showing that both the dimeric and monomeric forms of π are needed for promoting ori-α-dependent plasmid maintenance (in ihfΔ cells) and that the dimeric mutant forms (R75A) that failed to bind to DNA, also failed to support (along with monomeric π) ori α activity. A, a, schematic representation of α-γ plasmid DNA substrate; b, high copy number plasmid that supplied π cop in trans; c, γ and α-γ reporter replicons. B, propagation of α-γ plasmids carrying wt π and π cop in normal to low concentrations, in both IHF⁺ and ihfΔ cells. C, propagation of γ and α-γ plasmids in IHF⁺ and ihfΔ cells when π cop was supplied in trans from the high copy pUC19 plasmid. D, failure of the dimeric π containing the R75A mutation that prevented iteron binding to complement π cop in sustaining plasmid propagation. E, image of an agarose gel showing that cells that contained high concentration of π cop in trans did propagate the α-γ reporter as a plasmid. F, model showing a looped structure in which two iteron-bound monomeric π are bridged by a single dimer that not only requires protein-protein interaction with π monomers but also interaction with DNA.