Phage T4-induced DNA breaks activate a tRNA repair-defying anticodon nuclease

Summary: The natural role of the conserved bacterial anticodon nuclease (ACNase) RloC is not known, but traits that set it apart from the homologous phage T4-excluding ACNase PrrC could provide relevant clues. PrrC is silenced by a genetically linked DNA restriction-modification (RM) protein and turned on by a phage-encoded DNA restriction inhibitor. In contrast, RloC is rarely linked to an RM protein, and its ACNase is regulated by an internal switch responsive to double-stranded DNA breaks. Moreover, PrrC nicks the tRNA substrate, whereas RloC excises the wobble nucleotide. These distinctions suggested that (i) T4 and related phage that degrade their host DNA will activate RloC and (ii) the tRNA species consequently disrupted will not be restored by phage tRNA repair enzymes that counteract PrrC. Consistent with these predictions we show that Acinetobacter baylyiRloC expressed in Escherichia coli is activated by wild-type phage T4 but not by a mutant impaired in host DNA degradation. Moreover, host and T4 tRNA species disrupted by the activated ACNase were not restored by T4's tRNA repair system. Nonetheless, T4's plating efficiency was inefficiently impaired by AbaRloC, presumably due to a decoy function of the phage encoded tRNA target, the absence of which exacerbated the restriction. RloC is a bacterial wobble nucleotide excising anticodon nuclease activated by dsDNA breaks (DSB). Host DNA degradation by the phage T4 endonuclease DenA activated an RloC species heterologously expressed in E. coli. Host and T4 tRNA species (red and blue, respectively) that were consequently disrupted were not restored by T4's tRNA repair enzymes (green). Presumably, RloC protects its natural hosts from T4-like phage infections unless overwhelmed by the phage tRNA species it targets.