Colicin D, which is synthesised 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 [1]. 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 [2].

1: Structural inhibition of the colicin D tRNase by the tRNA-mimicking immunity protein. Graille M, Mora L, Buckingham RH, van Tilbeurgh H, de Zamaroczy M; EMBO J. 2004;23:1474-1482. PMID:15014439

2: Crystal structure of colicin E3: implications for cell entry and ribosome inactivation. Soelaiman S, Jakes K, Wu N, Li C, Shoham M; Mol Cell. 2001;8:1053-1062. PMID:11741540