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Kluyveromyces lactis γ-toxin is a tRNA endonuclease that cleaves Saccharomyces cerevisiae , and between position 34 and position 35. All three substrate tRNAs carry a 5-methoxycarbonylmethyl-2-thiouridine (mcm ⁵ s ² U) residue at position 34 (wobble position) of which the mcm ⁵ group is required for efficient cleavage. However, the different cleavage efficiencies of mcm ⁵ s ² U ₃₄ -containing tRNAs suggest that additional features of these tRNAs affect cleavage. In the present study, we show that a stable anticodon stem and the anticodon loop are the minimal requirements for cleavage by γ-toxin. A synthetic minihelix RNA corresponding to the anticodon stem loop (ASL) of the natural substrate is cleaved at …