DNA polymerase I exonuclease activity?

Question

Does DNA polymerase I in bacteria use forward or reverse exonuclease activity to remove RNA primers? One of my books says it uses 5' to 3', but another says it uses 3' to 5' exonuclease activity. Update: I know RNase H can remove the primers, but the exonuclease activity of pol I can also remove them.

Answer 1

Exonuclease activity typically removes from the 3' hydroxyl end of the last nucleotide, "chewing back" toward the 5' phosphate end.

The polymerase activity would add nucleotides to the free 3' hydroxyl end, growing the new strand in the 5' to 3' direction. The polymerase activity moves forward in a 5' to 3' direction, whereas the exonuclease activity works as a "backspace" and moves "backward" in a 3' to 5' direction.

Perhaps mixing up the 2 activities of the enzyme is where you were confused on the direction?

Hope that helps!

Mark