Initiation of Escherichia coli minichromosome replication at oriC and at protein n'' recognition sites. Two modes for initiating DNA synthesis in vitro.

The start sites for leading and lagging DNA strands were determined in vitro with minichromosomes as templates. Fragments from replication intermediates were analyzed by hybridization to single-stranded probes. Leading strand synthesis in the counterclockwise direction was found to originate in or close to (position 248 to -44) the minimal origin. Complementary lagging strand synthesis started several positions to the left outside of oriC. The results suggest in addition a concerted synthesis of leading and lagging strands following the dnaA directed assembly of initiation proteins at double-stranded oricC DNA (pre-replisome). In addition, DNA synthesis could initiate at protein n' recognition sequences located within and clockwise to the asnA gene. Initiation at n' sites was dependent on protein i activity, whereas leading and lagging strand initiation in the oriC region was not affected by protein i. Our results argue against an involvement of the phi X174-type primosome in the initiation of discontinuous DNA synthesis at oriC. An alternative function is suggested.