DNA TOPOISOMERASE AND RECOMBINASE ACTIVITIES IN NAE-I RESTRICTION-ENDONUCLEASE

Nae I endonuclease must bind to two DNA sequences for cleavage. Examin ation of the amino acid sequence of Nae I uncovered similarity to the active site of human DNA ligase I, except for leucine 43 in Nae I inst ead of the lysine essential for ligase activity. Changing leucine 43 t o lysine 43 (L43K) changed Nae I activity: Nae I-L43K relaxed supercoi led DNA to yield DNA topoisomers and recombined DNA to give dimeric mo lecules. Interruption of the reactions of Nae I and Nae I-L43K with DN A demonstrated transient protein-DNA covalent complexes. These finding s imply coupled endonuclease and ligase domains and link Nae I endonuc lease to the topoisomerase and recombinase protein families.