Primase
Encyclopedia
DNA primase is an enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 involved in the replication of DNA.

Primase catalyzes the synthesis of a short RNA (or DNA in some organisms ) segment called a primer
Primer (molecular biology)
A primer is a strand of nucleic acid that serves as a starting point for DNA synthesis. They are required for DNA replication because the enzymes that catalyze this process, DNA polymerases, can only add new nucleotides to an existing strand of DNA...

 complementary to a ssDNA template. Primase is of key importance in DNA replication
DNA replication
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule...

 because no known DNA polymerases can initiate the synthesis
Chemical synthesis
In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...

 of a DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 strand without an initial RNA or DNA primer
Primer (molecular biology)
A primer is a strand of nucleic acid that serves as a starting point for DNA synthesis. They are required for DNA replication because the enzymes that catalyze this process, DNA polymerases, can only add new nucleotides to an existing strand of DNA...

 (for temporary DNA elongation).

Function

In bacteria, primase binds to the DNA helicase forming a complex called the primosome
Primosome
In molecular biology, a primosome is a protein complex responsible for creating RNA primers on single stranded DNA during DNA replication.The primosome consists of seven proteins: DnaG primase, DnaB helicase, DnaC helicase assistant, DnaT, PriA, Pri B, and PriC...

. 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
DNA polymerase
A DNA polymerase is an enzyme that helps catalyze in the polymerization of deoxyribonucleotides into a DNA strand. DNA polymerases are best known for their feedback role in DNA replication, in which the polymerase "reads" an intact DNA strand as a template and uses it to synthesize the new strand....

.

The RNA segments are first elongated by DNA polymerase and then synthesized by primase. 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
Escherichia coli
Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

, 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 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
Helicase
Helicases are a class of enzymes vital to all living organisms. They are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two annealed nucleic acid strands using energy derived from ATP hydrolysis.-Function:Many cellular processes Helicases are a...

in viruses such as the T7 bacterialphage. 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.

External links

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