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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.

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From wikipedia:

In the replication process, RNAse H removes the RNA primer (created by Primase) from the lagging strand and then Polymerase I fills in the necessary nucleotides between the Okazaki fragments (see DNA replication) in 5' -> 3' direction...

And elsewhere in the same article:

Pol I possesses four enzymatic activities:

  1. A 5' -> 3' (forward) DNA-Dependent DNA polymerase activity, requiring a 3' primer site and a template strand
  2. A 3' -> 5' (reverse) exonuclease activity that mediates proofreading
  3. A 5' -> 3' (forward) exonuclease activity mediating nick translation during DNA repair.
  4. A 5' -> 3' (forward) RNA-Dependent DNA polymerase activity. Pol I operates on RNA templates with considerably lower efficiency (0.1–0.4%) than it does DNA templates, and this activity is probably of only limited biological significance.

So the answer to your question is neither. RNAse H removes the primers, and the exonuclease activity of DNA Pol I is irrelevant to primer removal, although it turns out it can operate in either direction depending on the context.

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Googling your question showed that it is RNAse H that removes the RNA primer. DNA Pol I has both 3'-5' as well as 5'-3' exonuclease activity for proofreading and nick repair activities, respectively. See the wiki on Pol I

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