Figure 9. Models describing how G4 might cooperate with a cis module to regulate the replication start site.

The orange box represents the cis element; the black arrows represent normal replication fork progression.

A, B The orientation of the G4 motif determines the positioning of the replication start site (green arrow) and that the position of the G4 motif with respect to its cooperating cis element is flexible.

C, D Two proposed models explaining how bimodal origins might be generated. One cis element surrounded by two G4 motifs in opposite orientations can induce two start sites (C). Two cis elements that cooperate with two G4 motifs in opposite orientations are required (D).