Cover: Flap endonucleases are exonucleases which also possess structure-specific endonuclease activity. The figure shows the backbone of one such enzyme from bacteriophage T5 (PDB code 1exn). Helical regions are shown in red, strands in yellow and other regions in blue. It is thought that single-stranded DNA can thread through the hole formed between two helices forming the inverted V above the active site. Garforth et al. in this issue [Nucleic Acids Res. (2001) 29, 2772-2779] show that the enzyme is able to use a wide range of divalent metal cofactors and possesses the ability to cleave double-stranded closed-circular DNA.
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