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The optional E. coli prr locus encodes a latent form of phage T4‐induced anticodon nuclease.

The optional Escherichia coli prr locus restricts phage T4 mutants lacking polynucleotide kinase or RNA ligase. Underlying this restriction is the specific manifestation of the T4‐induced anticodon nuclease, an enzyme which triggers the cleavage‐ligation of the host tRNALys. We report here the molecular cloning, nucleotide sequence and mutational analysis of prr‐associated DNA. The results indicate that prr encodes a latent form of anticodon nuclease consisting of a core enzyme and cognate masking agents. They suggest that the T4‐encoded factors of anticodon nuclease counteract the prr‐encoded masking agents, thus activating the latent enzyme. The encoding of a tRNA cleavage‐ligation pathway by two separate genetic systems which cohabitate E. coli may provide a clue to the evolution of RNA splicing mechanisms mediated by proteins.