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5' to 3' Exonuclease Activity

 

5' to 3' Exonuclease Activity - Polymerase I can also remove nucleotides in the 5'-->3' direction. This 5'à3' exonuclease activity has an important role in the removal of thymine dimers. Exposure of DNA to ultraviolet light may result in covalent linkage of adjacent pyrimidines (e.g. T=T) to form a pyrimidine dimer.

Since such a dimer cannot fit into the double helix it blocks replication unless removed. The 5'-->3' exonuclease activity of polymerase I excises the pyrimidine dimer region. Polymerase activity now enables repair synthesis.

Synthesis of DNA takes place in small fragments (Okazaki pieces) to each of which is attached a primer RNA segment. The 5'-->3' exonuclease activity is also responsible for removing the RNA segment and then filling in the gap by deoxyribonucleotides. As polymerase I moves ahead it cuts off ribonucleotides in front and adds deoxyribonucleotides behind.

Trypsin splits the polymerase I polypeptide chain into a large fragment (MW 75,000) and a small fragment (MW 36,000). The large fragment has polymerase as well as 3'-->5' exonuclease activity, while the smaller fragment has 5'-->3' exonuclease activity. It will thus be seen that polymerase I has synthetic (polymerization) activity as well as breakdown or degradation (exonuclease) activity.
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