Determinants of the cytotoxicity of PrrC anticodon nuclease and its amelioration by tRNA repair.

Breakage of tRNA(Lys(UUU)) by the Escherichia coli anticodon nuclease PrrC (EcoPrrC) underlies a host antiviral response to phage T4 infection that is ultimately thwarted by a virus-encoded RNA repair system. PrrC homologs are prevalent in other bacteria, but their activities and substrates are not defined. We find that induced expression of EcoPrrC is toxic in Saccharomyces cerevisiae and E. coli, whereas the Neisseria meningitidis PrrC (NmePrrC) is not. PrrCs consist of an N-terminal NTPase module and a C-terminal nuclease module. Domain swaps identified the EcoPrrC nuclease domain as decisive for toxicity when linked to either the Eco or Nme NTPase. Indeed, a single arginine-to-tryptophan change in the NmePrrC nuclease domain (R316W) educed a gain-of-function and rendered NmePrrC toxic to yeast, with genetic evidence for tRNA(Lys(UUU)) being the relevant target. The reciprocal Trp-to-Arg change in EcoPrrC (W335R) abolished its toxicity. Further mutagenesis of the EcoPrrC nuclease domain highlighted an ensemble of 15 essential residues and distinguished between hypomorphic alleles and potential nuclease-nulls. We report that the RNA repair phase of the bacterial virus-host dynamic is also portable to yeast, where coexpression of the T4 enzymes Pnkp and Rnl1 ameliorated the toxicity of NmePrrC-R316W. Plant tRNA ligase AtRNL also countered NmePrrC-R316W toxicity, in a manner that depended on AtRNL's 5'-kinase and ligase functions.