Functional analyses for tRNase Z variants: An aspartate and a histidine in the active site are essential for the catalytic activity

We performed functional analyses for various single amino-acid substitution variants of Escherichia coli, Bacillus subtilis, and human tRNase Zs. The well-conserved six histidine, His(I)–His(VI), and two aspartate, Asp(I) and Asp(II), residues together with metal ions are thought to form the active site of tRNase Z. The Mn2+-rescue analysis for Thermotoga maritima tRNase ZS has suggested that Asp(I) and His(V) directly contribute the proton transfer for the catalysis, and a catalytic mechanism has been proposed. However, experimental evidence supporting the proposed mechanism was limited. Here we intensively examined E. coli and B. subtilis tRNase ZS variants and human tRNase ZL variants for cleavage activities on pre-tRNAs in the presence of Mg2+ or Mn2+ ions. We observed that the Mn2+ ions cannot rescue the activities of Asp(I)Ala and His(V)Ala variants from each species, which are lost in the presence of Mg2+. This observation may support the proposed catalytic mechanism.