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Summary
We analysed the enzymatic activity (strand dis‐placement) of the Escherichia coli DnaB helicase on a mirror‐image pair of oligonucleotide‐based substrates mimicking the unwound replication origin oriC. Loading of the helicase complex occurred exclusively to the single‐stranded ‘lower strand’ part of the substrates. Full helicase activity required DnaA bound to the double‐stranded part of the substrates (oriC DnaA box R1) and to their single‐stranded ‘upper strand’ part. We assume that in vivo DnaA also loads the first of two helicase complexes – required for the assembly of two replication forks – to the lower strand of oriC during initiation of bidirectional chromosome replication in E. coli.