Crystal structure of NaeI - an evolutionary bridge between DNA endonuclease and topoisomerase

NaeI is transformed from DNA endonuclease to DNA topoisomerase and recombin ase by a single amino acid substitution. The crystal structure of NaeI was solved at 2.3 Angstrom resolution and shows that NaeI is a dimeric molecule with two domains per monomer. Each domain contains one potential DNA recog nition motif corresponding to either endonuclease or topoisomerase activity . The N-terminal domain core folds like the other type II restriction endon ucleases as well as lambda-exonuclease and the DNA repair enzymes MutH and Vsr, implying a common evolutionary origin and catalytic mechanism. The C-t erminal domain contains a catabolite activator protein (CAP) motif present in many DNA-binding proteins, including the type IA and type II topoisomera ses, Thus, the NaeI structure implies that DNA processing enzymes evolved f rom a few common ancestors, NaeI may be an evolutionary bridge between endo nuclease and DNA processing enzymes.