Colicins E5 and D cleave the anticodon loops of distinct tRNAs of Escherichia coli both in vivo and in vitro, which accounts for their bactericidal actions through depletion of tRNAs and prevention of protein synthesis. The targets of colicin E5 are five tRNA species for four amino acids, tyrosine, histidine, asparagine and aspartic acid, and those of colicin D are four isoaccepting tRNAs for arginine. These two colicins represent a new class, the "tRNase-type", of the nuclease-type colicins, which previously comprised the DNase-type and ribotoxin-type (or rRNase-type). On the other hand, a certain clinical E. coli strain produces a potentially suicidal "anticodon-nuclease", PrrC, in response to phage T4 infection, which specifically cleaves its own lysine tRNA. For these three tRNases, i.e. colicins E5 and D, and PrrC, the substrates and reaction products, as well as their physiological consequences, are very similar to each other, but so many molecular features are different that these three proteins are assumed to have acquired similar functions through evolutionary convergence from different origins.