A cytotoxic ribonuclease which specifically cleaves four isoaccepting arginine tRNAs at their anticodon loops

infected Escherichia coil cells by inactivating ribosomes, just like colici n E3. Here, we show that colicin D specifically cleaves tRNAs(Arg) includin g four isoaccepting molecules both in vivo and in vitro. The cleavage occur s in vitro between positions 38 and 39 in an anticodon loop with a 2'.3'-cy clic phosphate end, and is inhibited by a specific immunity protein. Consis tent with the cleavage of tRNAs(Arg) the RNA fraction of colicin-treated ce lls significantly reduced the amino acid-accepting activity only for argini ne. Furthermore. we generated a single mutation of histidine in the C-termi nal possible catalytic domain, which caused the loss of the killing activit y in vivo together with the tRNA(Arg)-cleaving activity both in vivo and in vitro. These findings show that colicin D directly cleaves cytoplasmic tRN As(Arg), which leads to impairment of protein synthesis and cell death. Rec ently. we found that colicin E5 stops protein synthesis by cleaving the ant icodons of specific tRNAs for Tyr. His, Asn. and Asp. Despite these apparen tly similar actions on tRNAs and cells, colicins D and E5 not only exhibit no sequence homology but also have different molecular mechanisms as to bot h substrate recognition and catalytic reaction.