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?