Cleavage of colicin D is necessary for cell killing and requires the innermembrane peptidase LepB

Colicin D is known to kill target cells by cleaving tRNA(Arg). A colicin D- resistant mutant was selected that was altered in the inner membrane leader peptidase, LepB. The substituted residue (Asn274Lys) is located close to t he catalytic site. The mutation abolishes colicin D cleavage but not the pr ocessing of exported proteins. LepB is required for colicin D cleavage, rel easing a small C-terminal fragment that retains full tRNase activity. The i mmunity protein was found to prevent colicin D processing and furthermore m asks tRNase activity, thus protecting colicin D against LepB-mediated cleav age during export. Catalytic colicins share a consensus sequence at their p utative processing site. Mutations affecting normal processing of colicin D abolish cytotoxicity without affecting the in vitro tRNase activity.