Primase

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 families: bacterial and archaeal/eukaryotic nuclear. Bacterial primases are monomers consisting of three domains. The N-terminal domain has a zinc-finger motif and is likely responsible for the initiation specificity of this enzyme. The central catalytic domain binds single-stranded DNA and catalyzes RNA polymer initiation and elongation complementary to it. The C-terminal domain interacts with other proteins, including DnaB helicase so that its activity takes place at 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 in the lowest amount. Archaeal/eukaryotic primase resides in a heterotetramer consisting of a small primase subunit, a large primase subunit, a regulatory phosphoprotein, and DNA polymerase alpha. The small subunit has primer synthesis activity that is modulated by the other three proteins in the complex as well as by Replication protein A, a single-stranded DNA-binding protein required for lagging strand DNA synthesis, and the GINS complex, 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 and also interacts with the DNA polymerase alpha/primase complex on the lagging strand.