Review
Anticodon nucleases

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Abstract

A tRNALys-specific anticodon nuclease is kept in a latent form in a rare Escherichia coli strain, complexed with a DNA restriction enzyme. A phage T4 inhibitor of DNA restriction activates anticodon nuclease, but other T4 proteins restore tRNALys. Detection of a homologous system in Neisseria and a different anticodon nuclease in colicin E5 suggest ubiquity and diversity of such tRNA toxins. Analysis of these systems could reveal novel RNA recognition and cleavage mechanisms.

Section snippets

Coupled DNA and tRNA restriction systems of latent ACNase holoenzyme

The optional E. coli prr locus (Fig. 2a) that specifies the latent ACNase was originally defined by the restriction of T4 pnk and rli mutants5, 10. It comprises the ACNase core gene prrC and three flanking genes whose products mask the activity of PrrC: prrAB/HsdMS, which are found upstream, and prrD/hsdR, which is downstream7, 11. Overexpression of prrC, either in E. coli12 or in human HeLa cells13, elicits cleavage of cellular tRNALys. The activity in human cells indicates that PrrC is the

T4 Stp: double-edged effector of coupled DNA and tRNA restriction systems

The meaning of the genetic linkage and physical coupling of the DNA and tRNA restriction systems of prr is hinted at by the dual functions of phage T4 stp. This minuscule gene was first characterized by mutations that suppress prr restriction8, 10, 19. Later, its product was shown to be necessary and sufficient for ACNase activation20, 21, 22. Thus, ACNase can be activated by expressing the 26-codon stp open reading frame in uninfected E. coli prr+ or by adding a synthetic Stp-like peptide to

Functional organization of the ACNase core

PrrC, the 396-residue core ACNase polypeptide, recognizes and cleaves tRNALys and also undergoes a reversible masking interaction with the DNA R–M enzyme EcoprrI. General considerations and certain experimental facts underlie a working hypothesis that assigns the two functions to different portions of PrrC (Fig. 4). A P-loop motif typical of ATP/GTP-binding domains containing ∼200 amino acids is found near the N terminus of the polypeptide12. Therefore, its first half might constitute such a

Homologous and analogous ACNases

A homologue of the latent ACNase locus prr has been detected in the pathogenic bacterium Neisseria meningitidis strain MC58. It comprises prrC flanked by hsd genes, both elements showing 60–70% amino acid identity with the E. coli counterparts (except for the variable regions of hsdS). Another N. meningitidis strain belonging to a different serogroup encodes an almost identical hsd locus that contains the typical hsdShsdR spacer instead of prrC (Ref. 27). These findings suggest ancient

Are ACNases classical protein enzymes or co-ribozymes?

PrrC and colicin E5 ACNases yield cyclic 2′,3′-phosphate and 5′-OH cleavage termini, as do tRNA splicing endonucleases9 and self-cleaving ribozymes29. ‘Classical’ RNases such as RNases A and T1 also generate these termini but proceed to hydrolyse the cyclic phosphodiester intermediate. Each of the two RNases utilizes a specific His residue as general acid catalyst; replacement or modification of this residue abolishes the activity30. Whether the ACNases have a similar or analogous catalytic

Possible applications

The two prototype ACNases are directed against conserved cues within key cellular targets. As such, they might prove useful in biomedical and biotechnological applications, such as being directed against parasitic microorganisms, virus-infected cells, cancerous cells and specialized tissues. In principle, the stable colicin E5 ACNase could be applied as such or in the form of a targeted fusion protein; the unstable PrrC ACNase might be advantageous for localized and temporal applications, such

Conclusions and outlook

The known ACNases might be examples of a widespread and versatile family of potentially useful RNA restriction enzymes. Despite the diverse biological contexts in which they operate, the two proteins might recognize and induce the cleavage of their tRNA ligands in a similar fashion. According to one of the models considered above, they are representatives of an era in which proteins began to take over the role of ribozymes, that of enhancing the self-cleaving potential of the RNA ligand without

Acknowledgements

The author is indebted to Thomas Bickle, Hyouta Himeno, Larry Snyder and members of his laboratory for critical reading of the manuscript and useful suggestions. Relevant work in the author’s laboratory is supported by grants from the Basic Science Foundation, the Israeli Academy of Sciences and Israeli Ministry of Health.

References (33)

  • R. Meidler

    Detection of anticodon nuclease residues involved in tRNALys cleavage specificity

    J. Mol. Biol.

    (1999)
  • Y. Nakamura

    Emerging understanding of translation termination

    Cell

    (1996)
  • H. Grosjean

    Studies of the complex between transfer RNAs with complementary anticodons. I. Origins of enhanced affinity between complementary triplets

    J. Mol. Biol.

    (1976)
  • M.B. Yarmolinsky

    Programmed cell death in bacterial populations

    Science

    (1995)
  • D.H. Parma

    The Rex system of bacteriophage lambda: tolerance and altruistic cell death

    Genes Dev.

    (1992)
  • Y.T. Yu et al.

    Translation elongation factor Tu cleaved by a phage-exclusion system

    Proc. Natl. Acad. Sci. U. S. A.

    (1994)
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