In E. coli cell, replication of genome starts at the single origin oriC. OriC is an AT rich sequence, composed of  232 – 245 bp. At the beginning, initiator protein DnaA will recognize and bind to DnaA boxes of oriC which contains 9 bp repeats, to form a large protein – DNA complex. Among the DnaA boxes, 3 boxes R1, R2, R4 have highest affinity with DnaA. This complex contributes to melting of A-T rich DNA unwinding element (DUE) which consists of three 13 bp repeats. The unwinding of DUE recruits helicase DnaB, with help of helicase loader DnaC. After that, helicase will open the double helix and recruit primase DnaG come to synthesize some RNA primer. RNA primer formation enables binding of Pol III holoenzyme to form 2 replication forks run in opposite directions from OriC.

Figure 1: Progression of replication initiation. Photo from Alberts B, Johnson A, Lewis J, et al, “The initiation and completion of DNA replication in chromosome”, Molecular biology of the cell, 4th edition.

This is a short overview of replication initiation in E. coli cell. You can refer these literature for more information.

Alberts B, Johnson A, Lewis J, et al. The initiation and completion of DNA replication in chromosome. Molecular biology of the cell, 4th edition.

Melissa L. Mott & James M. Berger. DNA replication initiation: mechanisms and regulation in bacteria. Nature Reviews Microbiology 5.