Flap endonucleases (FENs) are divalent metal ion-dependent phosphodiesterases. Metallonucleases are often assigned a "two-metal ion mechanism" where both metals contact the scissile phosphate diester. The spacing of the two metal ions observed in T5FEN structures appears to preclude this mechanism. However, the overall reaction catalyzed by wild type (WT) T5FEN requires three Mg 2؉ ions, implying that a third ion is needed during catalysis, and so a two-metal ion mechanism remains possible. , essential in all life forms, are members of the 5Ј-nuclease superfamily of structure-specific phosphate diesterases that carry out essential divalent metal ion-dependent nucleic acid hydrolysis (1). FENs act upon 5Ј-bifurcated structures known as flaps formed during lagging strand DNA replication and long patch base excision repair. Hydrolysis predominantly occurs one nucleotide into the 5Ј-duplex adjoining the site of bifurcation. An unusual feature of these enzymes is their high density of active site carboxylates that coordinate essential divalent metal ion cofactors. Bacterial and bacteriophage FENs possess eight active site acidic residues, seven of which are conserved in FENs from higher organisms and also in other members of the 5Ј-nuclease superfamily (1-4). In contrast, a typical metallonuclease active site consists of three or four carboxylates (5). However, a subclass of FEN paralogues, typified by Escherichia coli ExoIX, exists in eubacteria and appear to lack three of the metal-binding carboxylates (6).Although the most common mechanism suggested for metal-dependent phosphoryl transferases involves two metal ions in contact with the scissile phosphate diester, this is not universally accepted (5, 7-10). Debate about the validity or otherwise of two-metal ion catalysis has focused on several enzymes including flap endonucleases (11-16). Crystallographic studies of substrate-free FENs have demonstrated two active site-bound metal ions (14,(17)(18)(19). Metal ion 1 (M1) occupies a similar position in all structures. However, there is some variation in the position of the site coordinating a second metal ion (M2), often observed in FEN crystal structures. The distance between the M1 and M2 sites varies, and the separations observed are generally much greater than the 4 Å required for both metals to contact the same phosphate diester. For example, in bacteriophage T5FEN, 5 the M1-M2 separation of two Mn 2ϩ ions is 8 Å (Fig. 1A) (17). However, in a substrate-free structure of hFEN1, two magnesium ions are bound with a separation of 3.4 Å in one of three proteins bound to proliferating cell nuclear antigen (Fig. 1B) , 3-(N-morpholino)propane-sulfonic acid. 5 In early literature, T5FEN was referred to as T5 5Ј-3Ј-exonuclease and T5 5Ј-nuclease. In early literature, T4FEN was referred to as referred to also as T4 RNase H.