DNA primase is an enzyme involved in the replication of DNA. DNA primase is, in fact, a type of RNA polymerase which creates a RNA primer (later this RNA piece is removed by a 5' to 3' exonuclease); next, DNA polymerase uses the RNA primer to replicate ssDNA.
Primase catalyzes the synthesis of a short RNA (or DNA in some organisms ) segment called a primer complementary to a ssDNA template. Primase is of key importance in DNA replication because no known DNA polymerases can initiate the synthesis of a DNA strand without an initial RNA or DNA primer (for temporary DNA elongation).
In bacteria, primase binds to the DNA helicase forming a complex called the primosome. Primase is activated by DNA helicase where it then synthesizes a short RNA primer approximately 11 ±1 nucleotides long, to which new nucleotides can be added by DNA polymerase.
The RNA segments are first synthesized by primase and then elongated by DNA polymerase. Then the DNA polymerase forms a protein complex with two primase subunits to form the alpha DNA Polymerase primase complex. Primase is one of the most error prone and slow polymerases. Primases in organisms such as E. coli, synthesize around 2000 to 3000 primers at the rate of one primer per second. Primase also acts as a halting mechanism to prevent the leading strand from outpacing the lagging strand by halting the progression of the replication fork. The rate determining step in primase is when the first phosphodiester bond is formed with the ssDNA. The crystal structure of primase in E. coli with core that contained the DnaG protein was determined in 2000. The DnaG and primase complex is cashew shaped and contains three subdomains. The central subdomain forms a toprim fold which is made of a mixture five beta sheets and six alpha helices. The toprim fold is used for binding regulators and metals. The primase uses a phosphotransfer domain for the transfer coordination of metals, which makes it distinct from other polymerases. The side subunits contain a NH2 and COOH terminal made of alpha helixes and beta sheets. The NH2 terminal interacts with a zinc binding domain and COOH-terminal region which interacts with DnaB-ID. The replications mechanisms differ between different bacteria and viruses where the primase covalently link to helicase in viruses such as the T7 bacteriophage. In viruses such as herpes simplex virus (HSV-1), primase can form complexes with helicase. The primase-helicase complex is used to unwind dsDNA and synthesizes the lagging strand using RNA primers The majority of primers synthesized by primase are two to three nucleotides long.
- Overview article on primase structure and function (1995)
- DNA Primase at the US National Library of Medicine Medical Subject Headings (MeSH)
- Proteopedia: Helicase-binding domain of Escherichia coli primase
- Proteopedia: Complex between the DnaB helicase and the DnaG primase
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