High-fidelity DNA replication depends on both accurate incorporation of 
nucleotides in the newly synthesized strand and the maturation of Okazaki 
fragments. In eukaryotic cells, the latter is accomplished by a series of 
coordinated actions of a set of structure-specific nucleases, which, with the 
assistance of accessory proteins, recognize branched RNA/DNA configurations. In 
the current model of Okazaki fragment maturation, displacement of a 
27-nucleotide or longer flap is envisioned to attract replication protein A 
(RPA), which inhibits flap endonuclease-1 (FEN-1) but stimulates Dna2 nuclease 
for cleavage. Dna2 cleavage generates a short flap of 5-7 nucleotides, which 
resists binding by RPA and further cleavage by Dna2. FEN-1 then removes the 
remaining flap to produce a suitable substrate for ligation. However, FEN-1 is 
not efficient in cleaving the short flap, and we therefore set out to identify 
cellular factors that might regulate FEN-1 activity. Through 
co-immunoprecipitation experiments, we have isolated heterogeneous nuclear 
ribonucleoprotein A1 (hnRNP A1), which forms a direct complex with FEN-1 and 
stimulates its enzymatic activities. The stimulation by hnRNP A1 is most 
dramatic using DNA substrates with short flaps. With longer flap substrates the 
hnRNP A1 effect is more modest and is suppressed by the addition of RPA. A model 
is provided to explain the possible in vivo role of this interaction and 
activity in Okazaki fragment maturation.