The 5'-3' exonuclease activity is the only active component of the N-terminus fragment of DNA Polymerase I. The main duty of the 5'-3' exonuclease activity is to remove the RNA primers at the 5' ends of newly synthesized DNA so that the polymerase activity can fill in the resulting gaps.
It is also important to note that DNA Polymerase I is the only E. coli DNA polymerase that has the 5'-3' exonuclease active site. Both Pol. II and Pol. III can polymerize DNA and excise fragments in the 3'-5' direction, but neither of them is capable of this functionality.
The N-terminus fragment is a much smaller fragment than the Klenow fragment. In fact, it constitutes residues 1-323, while residues 324-928 compose the larger section (10). Proteases such as substilin and trypsin can cleave the Pol. I into these two fragments. Thus, despite the fact that we generally discuss Pol. I according to different fragments, it nevertheless is a one-subunit enzyme with three different activities. Pol. III, on the other hand, has a core consisting of 7 subunits which functions as part of a complex multisubunit enzyme, Pol. III holoenzyme, which consists of at least 10 types of subunits (9).
Below is an example of the exonuclease 5'-3' activity. It generally excises short oligomers between 1 and 10 nucleotides long:
Fig. 2: This is a basic, inexcusably blurry diagram of the exonuclease activity in the 5'-3' direction. DNA strands are shortened by removal of nucleotides from the 5' end of the molecule. At least it's in color...