Replisome assembly at oriC, the replication origin of E. coli, reveals an explanation for initiation sites outside an origin

Mol Cell. 1999 Oct;4(4):541-53. doi: 10.1016/s1097-2765(00)80205-1.


This study outlines the events downstream of origin unwinding by DnaA, leading to assembly of two replication forks at the E. coli origin, oriC. We show that two hexamers of DnaB assemble onto the opposing strands of the resulting bubble, expanding it further, yet helicase action is not required. Primase cannot act until the helicases move 65 nucleotides or more. Once primers are formed, two molecules of the large DNA polymerase III holoenzyme machinery assemble into the bubble, forming two replication forks. Primer locations are heterogeneous; some are even outside oriC. This observation generalizes to many systems, prokaryotic and eukaryotic. Heterogeneous initiation sites are likely explained by primase functioning with a moving helicase target.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Bacterial Proteins / genetics
  • Bacterial Proteins / metabolism
  • Base Sequence
  • DNA / metabolism
  • DNA Footprinting
  • DNA Helicases / genetics
  • DNA Helicases / metabolism
  • DNA Polymerase III / metabolism
  • DNA Primase / metabolism
  • DNA Replication / genetics*
  • DnaB Helicases
  • Escherichia coli / genetics*
  • Molecular Sequence Data
  • Mutation
  • Plasmids
  • RNA / metabolism
  • Replication Origin / genetics*
  • Replicon / genetics


  • Bacterial Proteins
  • RNA
  • DNA
  • DNA Primase
  • DNA Polymerase III
  • DNA Helicases
  • DnaB Helicases