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

Abstract

Colicin D has long been thought to stop protein synthesis in infected Escherichia coli cells by inactivating ribosomes, just like colicin E3. Here, we show that colicin D specifically cleaves tRNAs Arg including four isoaccepting molecules both in vivo and in vitro. The cleavage occurs in vitro between positions 38 and 39 in an anticodon loop with a 2 ,3 -cyclic phosphate end, and is inhibited by a specific immunity protein. Consistent with the cleavage of tRNAs Arg , the RNA fraction of colicin-treated cells significantly reduced the amino acid-accepting activity only for arginine. Furthermore, we generated a single mutation of histidine in the C-terminal possible catalytic domain, which caused the loss of the killing activity in vivo together with the tRNA Arg -cleaving activity both in vivo and in vitro. These findings show that colicin D directly cleaves cytoplasmic tRNAs Arg , which leads to impairment of protein synthesis and cell death. Recently, we found that colicin E5 stops protein synthesis by cleaving the anticodons of specific tRNAs for Tyr, His, Asn, and Asp. Despite these apparently similar actions on tRNAs and cells, colicins D and E5 not only exhibit no sequence homology but also have different molecular mechanisms as to both substrate recognition and catalytic reaction.C olicins are plasmid-encoded proteins that are toxic to Escherichia coli cells that do not have the same plasmid or a cognate Col plasmid (1-5). Most colicins are produced in response to SOS-inducing signals and are secreted into the medium. After binding to cell-surface receptors on sensitive cells, they are translocated across the membrane and then exert their final cytotoxic activities, which are attributable to their C-terminal domains. Two major modes of toxicity are well known; colicins A, B, E1, Ia, Ib, K, and N are ion-channel formers attacking the cytoplasmic membrane, and colicins E2 to E9 are nucleases. In the latter group, E2, E7, E8, and E9 are DNases, and E3 is a special kind of RNase that cleaves 16S rRNA within ribosomes.Colicins E4 to E6 quickly stop amino acid incorporation in treated cells (6), suggesting impairment of protein synthesis analogous in the established case of E3, which specifically cleaves 16S-RNA at the 49th bond from the 3Ј end leading to inactivation of ribosomes (7-9). This is also the case with colicin D, which Timmis and Hedges have characterized as to the physiological response of treated cells (10, 11), although the actual molecular basis of the cytotoxic effect of colicin D, as well as those of E4 to E6, remained to be elucidated. Colicins E4 and E6 proved to be E3-homologs and showed comparable activity toward ribosomes (ref. 12; GenBank accession number X63621; Y. Gunji, M. Ohno, T.O., H.M., and T.U., unpublished data), but colicin E5 exhibits no similarity to E3 in the C-terminal active domain. We recently showed that colicin E5 comprises a third category of nuclease-type colicins, which does not attack ribosomes but specific tRNAs (13). E5 is a novel RNase that cleaves the anticodons...

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