A specific fragment of the phi X174 viral circle sustains the primed start of complementary DNA strand synthesis in vitro, even though the intact circle permits primed starts at many sites. The 300-nucleotide fragment from restriction nuclease digestion contains the recognition site for protein n', a DNA-dependent ATPase essential for priming phi X174 DNA replication. This n' recognition site contains within it a 44-nucleotide sequence with a potential hairpin structure and may be regarded as the starting signal for replication [Shlomai, J. & Kornberg, A. (1980) Proc. Natl. Acad. Sci. USA 77, 799-803]. After initiation on the 3' side of this sequence, the priming system (primosome) repeatedly generates primers by moving processively on the DNA template in a direction opposite to chain elongation. This primosome mobility is an attractive model for the discontinuous phase of Escherichia coli chromosome replication, in which processive primosome movement with the replicating fork is proposed for repeated initiations of nascent replication fragments.

Arai, K., & Kornberg, A. (1981). Unique primed start of phage phi X174 DNA replication and mobility of the primosome in a direction opposite chain synthesis. Proceedings of the National Academy of Sciences, 78(1), 69–73.