Initiation of enzymatic replication at the origin of the Escherichia coli chromosome: primase as the sole priming enzyme.

The enzymatic replication of plasmids containing the unique (245 base pair) origin of the Escherichia coli chromosome (oriC) can be initiated with any of three enzyme priming systems: primase alone, RNA polymerase alone, or both combined (Ogawa, T., Baker, T. A., van der Ende, A. & Kornberg, A. (1985) Proc. Natl. Acad. Sci. USA 82, 3562-3566). At certain levels of auxiliary proteins (topoisomerase I, protein HU, and RNase H), the solo primase system is efficient and responsible for priming synthesis of all DNA strands. Replication of oriC plasmids is here separated into four stages: (i) formation of an isolable, prepriming complex requiring oriC, dnaA protein, dnaB protein, dnaC protein, gyrase, single-strand binding protein, and ATP; (ii) formation of a primed template by primase; (iii) rapid, semiconservative replication by DNA polymerase III holoenzyme; and (iv) conversion of nearly completed daughter molecules to larger DNA forms. Optimal initiation of the leading strand of DNA synthesis, over a range of levels of auxiliary proteins, appears to depend on transcriptional activation of the oriC region by RNA polymerase prior to priming by primase.