The Pol α-primase complex.

Initiation of DNA synthesis in eukaryotic replication depends on the Pol α-primase complex, a multi-protein complex endowed with polymerase and primase activity. The Pol α-primase complex assembles the RNA-DNA primers required by the processive Pol δ and Pol ε for bulk DNA synthesis on the lagging and leading strand, respectively. During primer synthesis, the primase subunits synthesise de novo an oligomer of 7-12 ribonucleotides in length, which undergoes limited extension with deoxyribonucleotides by Pol α. Despite its central importance to DNA replication, little is known about the mechanism of primer synthesis by the Pol α-primase complex, which comprises the steps of initiation, 'counting' and hand-off of the RNA primer by the primase to Pol α, followed by primer extension with dNTPs and completion of the RNA-DNA hybrid primer. Recent biochemical and structural work has started to provide some insight into the molecular basis of initiation of DNA synthesis. Important advances include the structural characterisation of the evolutionarily related archaeal primase, the elucidation of the mechanism of interaction between Pol α and its B subunit and the observation that the regulatory subunit of the primase contains an iron-sulfur cluster domain that is essential for primer synthesis.