Figure 2.

(A) Model of staged E. coli orisome assembly. Stage1: After oriC is replicated, SeqA (red octagons) binds to hemimethylated 5′-GATC motifs, and DnaA rebinds to high affinity R1, R2, and R4 sites. Fis (green triangle) is also bound. DnaA and Fis binding constrains the origin, possibly through interactions among the bound DnaA molecules (indicated by dotted lines), and prevents IHF from binding to its recognition site. Stage 2: DnaA bound to R4 recruits DnaA for binding to C1. DnaA-ATP then progressively fills the remaining sites in the right array. DnaA displaces Fis, and loss of Fis allows IHF to bind. Stage 3: The IHF-induced bend allows DnaA cooperative binding of DnaA between R1 and R5M. DnaA then progressively fills the remaining sites in the left array. Stage 4: The DUE is unwound and one of the single strands interacts with DnaA-ATP bound to the left array. The figure is adapted from Leonard and Grimwade (2015). (B) Possible mechanisms of Fis displacement by DnaA. When sufficient levels of DnaA accumulate, Fis is displaced either by direct competition with DnaA binding to C3 (left panel) or by a conformation change caused by DnaA binding to the right low affinity sites in oriC (right panel).