HSD restriction - Modification proteins partake in latent anticodon nuclease

Phage T4-induced anticodon nuclease triggers cleavage-ligation of the host tRNA^Lys. The enzyme is encoded in latent form by the optional Escherichia coli locus prr and is activated by the product of the phage stp gene. Anticodon nuclease latency is attributed to the masking of the core function prrC by flanking elements homologous with type I restriction-modification genes (prrA-hsdM and prrD-hsdR). Activation of anticodon nuclease in extracts of uninfected prr⁺ cells required synthetic Stp, ATP and GTP and appeared to depend on endogenous DNA. Stp could be substituted by a small, heat-stable E.coli factor, hinting that anticodon nuclease may be mobilized in cellular situations other than T4 infection. Hsd antibodies recognized the anticodon nuclease holoenzyme but not the prrC-encoded core. Taken together, these data indicate that Hsd proteins partake in the latent ACNase complex where they mask the core factor PrrC. Presumably, this masking interaction is disrupted by Stp in conjunction with Hsd ligands. The Hsd-PrrC interaction may signify coupling and mutual enhancement of two prokaryotic restriction systems operating at the DNA and tRNA levels.