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DNA polymerase I (PolI) contains polymerase, 3'-5' exonuclease, and 5'-3' exonuclease activities in a single polypeptide chain of molecular weight 109 kDa. If E. coli Pol I is exposed to the protease subtilisin, the protein is cleaved into a large (75 kDa) fragment and a smaller (34 kDa) fragment. The large fragment has polymerase and 3'-5' exonuclease activity and the smaller fragment has 5'-3' exonuclease activity. The smaller fragment is found to have the same N-terminal amino acid sequence as the native (uncut) protein. What do these observations indicate about the structure of PolI?

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Is this homework? What exactly are you asking? –  terdon Feb 21 '13 at 19:14
I suspect this is homework, so I will give you a hint (I hope I understand the question correctly). Think about the following: Do you think all the functions of the polymerase are performed by the same set of amino acids? What is the relation between 1D and 3D structure? –  Bitwise Feb 22 '13 at 1:30
The first observation that can be made concerning the structure of PolI is that it contains domains. They are independently folded regions of the protein that each has different functions. They are connected by linkers. The information provided can lead to the conclusion that there are at least two domains that are functioning independently. Also, the protease would cut the linker between the domains. This would free the 5'-3' exonuclease activity that is present in the N-terminal domain. ---> This is what I believe is the correct answer. Thoughts? –  Abigailb55 Feb 22 '13 at 2:33

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