Effects on NaeI-DNA recognition of the leucine to lysine substitution that transforms restriction endonuclease NaeI to a topoisomerase: A model for restriction endonuclease evolution

Substituting lysine for leucine at position 43 (L43K) transforms Nael from restriction endonuclease to topoisomerase and makes Nael hypersensitive to intercalative anticancer drugs. Here we investigated DNA recognition by Nael-L43K. Using DNA competition and gel retardation assays, Nael-L43K showed reduced affinity for DNA substrate and the ability to bind both single- and double-stranded DNA with a definite preference for the former. Sedimentation studies showed that under native conditions Nael-L43K, like Nael, is a dimer. Introduction of mismatched bases into double-stranded DNA significantly increased that DNA's ability to inhibit Nael-L43K. Wild-type Nael showed no detectable binding of either single-stranded DNA or mismatched DNA over the concentration range studied. These results demonstrate that the L43K substitution caused a significant change in recognition specificity by Nad and imply that Nael-L43K's topoisomerase activity is related to its ability to bind single-stranded and distorted regions in DNA, A mechanism is proposed for the evolution of the Nael restriction-modification system from a topoisomerase/ligase by a mutation that abolished religation activity and provided a needed change in DNA recognition.