cryo-EM of type III restriction enzymes

Type III restriction enzymes recognize specific DNA sequences and cut at a short distance from the recognition site, requiring both ATP and S-adenosylmethionine (SAM) for activity. Unlike Type I and Type II enzymes, Type III restriction enzymes require two inversely oriented recognition sites on the DNA to cleave the target sequence. Type III restriction-modification systems function as multi-subunit complexes, consisting of both a restriction enzyme and a methyltransferase, which work together to protect the host DNA. They exhibit both nuclease and methylase activity, playing a role in the host defense mechanism against foreign DNA.The cleavage by Type III restriction enzymes is often unpredictable, occurring several bases away from the recognition site. Structural studies are lacking and our aim is to determine the cryo-EM structure of a model Type III restriction enzyme, called MBOIII with DNA substrates and elucidate the functional mechanism of the type III restriction enzyme.