Initiation of DNA replication at the Escherichia coli chromosomal origin occurs through an ordered series of events that depends first on the binding of DnaA protein, the replication initiator, to DnaA box sequences followed by unwinding of an AT-rich region. A step that follows is the binding of DnaB helicase at oriC so that it is properly positioned at each replication fork. We show that DnaA protein actively mediates the entry of DnaB at oriC. One region (amino acids 111-148) transiently binds to DnaB as determined by surface plasmon resonance. A second functional domain, possibly involving formation of a unique nucleoprotein structure, promotes the stable binding of DnaB during the initiation process and is inactivated in forming an intermediate termed the prepriming complex by removal of the N-terminal 62 residues. Based on similarities in the replication process between prokaryotes and eukaryotes, these results suggest that a similar mechanism may load the eukaryotic replicative helicase.