Primase

DNA primase is an RNAP enzyme involved in the replication of DNA.

Primase catalyzes the synthesis of a short RNA 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).

Function

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 elongated by DNA polymerase and then synthesized by primase.^[1] 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 polymerase. ^[2] Primases in organisms such as E coli, around 2000 to 3000 nucleotides at the rate of one nucleotide per second.^[3] 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. ^[4] The rate determining step in primase is when the first phosphodiester bond is formed with the ssDNA. ^[5] The crystal structure of primase in E. coli with core that contained the DnaG protein was determined in 2000. ^[6] The DnaG and primase complex is cashew shaped and contains three subdomains. ^[7] The central subdomain forms a toprim fold which is made of a mixture five beta sheets and six alpha helices. ^[8] The toprim fold is used for binding regulators and metals. The primase uses a phosphotrasfer domain for the transfer coordination of metals, which makes it distinct from other polymerases. ^[9] The side subunits contain a NH2 and COOH terminal made of made of alpha helixes and beta sheets. ^[10] The NH2 terminal interacts with a zinc binding domain and COOH-terminal region which interacts with DnaB-ID. ^[11] The replications mechanisms differ between different between bacteria and viruses where the primase covalently link to helicase in viruses such as the T7 bacterialphage. ^[12] In viruses such as herpes simple virus (HSV-1), primase can form complexes with helicase.^[13] The primase-helicase complex is used to unwind dsDNA and synthesizes the lagging strand using RNA primers.^[14] The majority of primers synthesized by primase are two to three nucleotides long.^[15]

Types

• dnaG

External links

• Overview article on primase structure and function (1995)
• MeSH DNA+Primase
• youtube clip put together by a professor's assistant

References