Colicin D, which is synthesized by various prokaryotes, adopts an antiparallel four helical bundle fold: the helices are tightly packed, forming a compact cylindrical molecule. The protein specifically cleaves the anticodon loop of all four tRNA-Arg isoacceptors, thereby inactivating prokaryotic protein synthesis and leading to cell death. This family also contains immunity proteins to klebicins and microcins. Many bacteria produce proteins that destroy their competitors. Colicin D is one such. The immunity proteins are expressed on the same operon as their cognate bacteriocins and protect the expressing bacterium from the effects of its own bacteriocin.