Figure 34.11 The origin of replication in E. coli and formation of the prepriming complex. (A) The oriC locus has a length of 245 bp. It contains a tandem array of three nearly identical AT-rich regions (green) and four binding sites (orange) for the DnaA protein. (B) Monomers of DnaA bind to their binding sites in oriC and oligomerize, wrapping the DNA around the oligomer. This structure marks the origin of replication and favors DNA strand separation in the AT-rich sites (green). (C) The AT-rich regions are unwound by DnaB and trapped by the single-stranded-binding protein (SSB). At this stage, the complex is ready for the synthesis of the RNA primers and assembly of the DNA polymerase III holoenzyme.