In vitro assembly of a prepriming complex at the origin of the Escherichia coli chromosome.

During initiation of DNA replication of plasmids containing the origin of the Escherichia coli chromosome (oriC), the proteins dnaA, dnaB, and dnaC interact and assemble a complex at oriC. The complex is larger and more asymmetric than that formed by dnaA protein and embraces an extra 50 base pairs at the left side of the minimal oriC sequence. Both dnaA and dnaB proteins have been identified in the complex by electron microscopy and antibody binding; dnaC protein was not detected. HU protein, which stimulates the activity of the initiation reaction, was often present. Entry of dnaB protein required dnaA and dnaC proteins and a supercoiled template. Thus, a complex structure, involving multiple proteins and a large region of DNA, must be formed at the origin to prepare the template for priming and replication.