Anticodon endonuclease (ACNase) that triggers the cleavage ligation of tRNA(Lys). It is activated by T4 stp protein and masked by the prrD protein (the endonuclease subunit of EcoprrI) (PubMed:1691706). The prr locus restricts phage T4 mutants lacking polynucleotide kinase or RNA ligase; T4 mutants lacking these genes manifest a T4-induced anticodon nuclease (ACNase). It is thought that Stp and other T4-encoded ACNase factors counteract the masking agents, thus activating the latent ACNase (Probable). Loss of ACNase activity.