Primase

Abstract

Primase is the enzyme that synthesizes RNA primers, oligonucleotides that are complementarily bound to a nucleic acid polymer. Primase is required because DNA polymerases cannot initiate polymer synthesis on single-stranded DNA templates; they can only elongate from the 3’-hydroxyl of a primer. Primases fall into two major sequence and structure superfamilies: bacterial and eukaryotic nuclear. Genes for both superfamilies were likely present in the Last Universal Common Ancestor but only one type retained its primase function in each lineage of cellular organisms. Bacterial and eukaryotic primases have a shared mechanism despite their non-homologous genes and different protein folds. Bacterial primases are monomers that interact with DnaB helicase, which unwinds the replication fork. The bacterial primase gene, dnaG, is the central gene of the macromolecular synthesis operon carrying the genes for the initiation phases of translation, replication, and transcription. Of the three genes, dnaG is under the most levels of control and is expressed at the lowest levels. Eukaryotic nuclear and archaeal primases are heterodimers consisting of a small primase subunit with primer synthesis activity and a large primase subunit that modulates its activity. In eukaryotes, these two proteins are also associated with a regulatory phosphoprotein, DNA polymerase alpha, replication protein A, and GINS, the central hub around which the leading and lagging strand DNA replicases assemble to control the progression of the replication fork. GINS interacts with the MCM helicase that translocates on the leading strand template but also interacts with the DNA polymerase alpha/primase complex on the lagging strand. In archaea, the eukaryotic-like heterodimer of small and large subunits is associated with proteins that modulate its activity but are not homologous to the eukaryotic proteins.