Dynamics of DNA polymerase III holoenzyme of Escherichia coli in replication of a multiprimed template.

Movements of DNA polymerase III holoenzyme (holoenzyme) in replicating a template multiprimed with synthetic pentadecadeoxynucleotides (15-mers) annealed at known positions on a single-stranded circular or linear DNA have been analyzed. After extension of one 15-mer on a multiprimed template, holoenzyme moves downstream in the direction of chain elongation to the next primer. Holoenzyme readily traverses a duplex, even 400 base pairs long, to exploit its 3'-hydroxyl end as the next available primer. This downstream polarity likely results from an inability to diffuse upstream along single-stranded DNA. These holoenzyme movements, unlike formation of the initial complex with a primer, do not require ATP. Time elapsed between completion of a chain and initiation on the next downstream primer is rapid (1 s or less); dissociation of holoenzyme to form a complex with another primed template is slow (1-2 min). Thus, holoenzyme diffuses rapidly only on duplex DNA, probably in both directions, and forms an initiation complex with the first primer encountered. Based on these findings, schemes can be considered for holoenzyme action at the replication fork of a duplex chromosome.