An Mrr-family nuclease motif in the single polypeptide restrictionmodification enzyme LlaGI

Bioinformatic analysis of the putative nuclease domain of the single polypeptide restriction– modification enzyme LlaGI reveals amino acid motifs characteristic of the Escherichia coli methylated DNA-specific Mrr endonuclease. Using mutagenesis, we examined the role of the con-served residues in both DNA translocation and cleavage. Mutations in those residues predicted to play a role in DNA hydrolysis produced enzymes that could translocate on DNA but were either unable to cleave the polynucleotide track or had reduced nuclease activity. Cleavage by LlaGI is not targeted to methylated DNA, suggesting that the conserved motifs in the Mrr domain are a conven-tional sub-family of the PD-(D/E)XK superfamily of DNA nucleases