Replication of pBR322 DNA in vitro with purified proteins. Requirement for topoisomerase I in the maintenance of template specificity.

The replication of plasmid pBR322 DNA has been reconstituted with purified proteins from Escherichia coli. Initiation of the leading-strand requires RNA polymerase holoenzyme, DNA polymerase I, RNase H, and DNA gyrase. Initiation of the lagging-strand requires the primosomal proteins (the dnaB, dnaC, and dnaG proteins, replication factor Y (protein n') and proteins i, n, and n") and the single-stranded DNA binding protein. DNA polymerase III holoenzyme is required for extensive elongation of the nascent DNA chains. The products of this replication reaction are primarily nonsegregated daughter molecules. However, the addition of small amounts of soluble extract from E. coli results in the completion and segregation of these molecules to give mature form I DNA, suggesting that additional factors are required for this process. Topoisomerase I is necessary to make the replication system specific for pBR322 DNA as a template, indicating that the linking number of the DNA, determined by an equilibrium between the opposing activities of topoisomerase I and DNA gyrase, plays a crucial role in determining the reactivity of the DNA molecule toward initiating DNA replication. The function of the proteins involved in the replication of this closed-circular, double-stranded, superhelical DNA is discussed.