E. coli DNA polymerase I, which is involved in DNA replication and repair, possess three distinct activities. The first of these activities is a DNA polymerase activity while the two other are exonuclease activities, one in the 3'-5' direction and one in the 5'-3' direction. The 3'-5' exonuclease activity allows incorrect nucleotides added by the DNA polymerase activity to be removed. The 5'-3' exonuclease activity allows removal of the old RNA primers.
The three distinct enzymatic functions of DNA pol I are segregated to the three domains. This multi-domain organisation insures that the three activities involved in DNA replication can be carried out efficiently, in the required order and that they are present in equivalent quantities in the cell. The Klenow fragment produced from DNA pol I by proteolytic cleavage corresponds to the two C-terminal domains which have 3'-5' exonuclease and DNA polymerase activities
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