RNA primer removal from Okazaki fragments during lagging-strand replication and 
the excision of damaged DNA bases requires the action of structure-specific 
nucleases, such as the mammalian flap endonuclease 1 (FEN-1). This nuclease 
contains two conserved motifs enriched with acidic amino acid residues that are 
important for catalytic function. Similar motifs have been identified in 
nucleases found in viruses, archebacteria, eubacteria, and in eukaryotes ranging 
from yeast to humans. Unique among these proteins, the putative FEN-1 homologue 
in Escherichia coli is contained within the N-terminal region of the DNA 
polymerase I (PolN). To demonstrate that the cellular functions of FEN-1 reside 
in PolN, we cloned and expressed the amino terminal domain (323 amino acid 
residues) of PolI in a Saccharomyces cerevisiae strain lacking the FEN-1 
homologue RAD27. Overexpression of PolN suppressed, to varying degrees, 
phenotypes associated with a rad27 null strain. These include temperature 
sensitivity, Okazaki fragment processing, a mutator phenotype, a G2/M cell cycle 
arrest, minichromosome loss, and methyl methane sulfonate sensitivity. We 
purified Rad27 and PolN proteins in order to determine whether differences in 
their intrinsic nuclease activities or interaction with proliferating cell 
nuclear antigen (PCNA) could explain the partial suppression of some phenotypes. 
We found that the in vitro nuclease activities of Rad27 were more potent than 
those of PolN and the activity of Rad27, but not PolN, was stimulated by PCNA. 
We conclude that the N-terminal nuclease domain of E. coli polymerase I encodes 
a functional homologue of FEN-1.