An oligomeric switch controls the Mrr-induced SOS response in E. coli

Mrr from Escherichia coli K12 is a type IV restriction endonuclease whose role is to recognize and cleave foreign methylated DNA. Beyond this protective role, Mrr can inflict chromosomal DNA damage that elicits the SOS response in the host cell upon heterologous expression of specific methyltransferases such as M.HhaII, or after exposure to high pressure (HP). Activation of Mrr in response to these perturbations involves an oligomeric switch that dissociates inactive homo-tetramers into active dimers. Here we used scanning number and brightness (sN&B) analysis to determine in vivo the stoichiometry of a constitutively active Mrr mutant predicted to be dimeric and examine other GFP-Mrr mutants compromised in their response to either M.HhaII activity or HP shock. We also observed in vitro the direct pressure-induced tetramer dissociation by HP fluorescence correlation spectroscopy of purified GFP-Mrr. To shed light on the linkages between subunit interactions and activity of Mrr and its variants, we built a structural model of the full-length tetramer bound to DNA. Similar to functionally related endonucleases, the conserved DNA cleavage domain would be sequestered by the DNA recognition domain in the Mrr inactive tetramer, dissociating into an enzymatically active dimer upon interaction with multiple DNA sites.