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Polymerase I - Pol I
 

Polymerase I - Pol I
The Kornberg enzyme, now called polymerase I, is no longer considered to be an essential enzyme for protein synthesis. Bacterial mutants lacking this enzyme are able to synthesize DNA.

It is believed to take part in the repair of DNA Polymerase I is a single polypeptide chain with a molecular weight of 109,000. There is one atom of Zn present per chain. Thus DNA polymerases are metalloenzymes. About 400 molecules of polymerase I are present in E. coli.

Electron microscope studies show that DNA polymerase is roughly spherical in shape with a diameter of about 65A. It is attached at regular intervals to the DNA chain. DNA polymerase contains a number of sites which have functional importance.

(i) A template site which attaches to the DNA template and holds a section of DNA in place.

(ii) A primer site which contains the primer, a short ( Approx.100 nucleotides) complementary segment of RNA on which the newly synthesized DNA strand grows.

(iii) A primer terminus site, at the tip of the primer, which has a terminal 3'OH group.

(iv) A triphosphate site where an incoming nucleoside triphosphate matches a complementary nucleotide on the DNA template and is bound to the 3'OH position of the primer.

Polymerase I of E. coli has three activities, polymerization activity,3'-->5' exonuclease activity and 5'-->3' exonuclease activity
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