A HELICAL ARCH ALLOWING SINGLE-STRANDED-DNA TO THREAD THROUGH T5 5'-EXONUCLEASE

THE 5'-exonucleases are enzymes that are essential for DNA replication and repair(1). As well as their exonucleolytic action, removing nucle otides from the 5'-end of nucleic acid molecules such as Okazaki fragm ents(2), many 5'-3'-exonucleases have been Shown to possess endonucleo lytic activities(3,4), T5 5'-3'-exonuclease shares many similarities w ith the amino termini of eubacterial DNA polymerases(5), although, unl ike eubacteria, phages such as T5, T4 and T7 express polymerase and 5' -exonuclease proteins from separate genes, Here we report the 2.5-Angs trom crystal structure of the phage T5 5'-exonuclease, which reveals a helical arch for binding DNA, We propose a model consistent with a th reading mechanism in which single-stranded DNA could slide through the arch, which is formed by two helices, one containing positively charg ed, and the other hydrophobic, residues. The active site is at the bas e of the arch, and contains two metal-binding sites.