[Some peculiarities of transfer RNAs hydrolysis by exonuclease A5].

Unusual kinetics of the hydrolysis of the baker's yeast total tRNA and tRNA1Val by exonuclease A5 was found (two stages of the reaction; low initial velocity and still lower velocity of the second reaction stage; high Km values). These peculiarities were shown to be due to the three-dimensional structure of the substrate, which makes a large part of the tRNA molecule resistant to the exonuclease A5 attack. The exhaustive tRNA hydrolysis observed in the presence of very large doses of exonuclease A5 may be explained by a slight (less than 1%) endo-RNAase contamination in the exonuclease A5 preparation. The hydrolysis conditions become optimal, i.e. the exhaustive hydrolysis proceeds most rapidly, when the amount of endo-RNAase admixture reaches 20--30%. The artificial enzyme mixture of such a composition is a convenient reagent for RNA and expecially for tRNA hydrolysis under mild conditions to 5'-mononucleotides. The considerable resistance of tRNA to pure exonuclease A5 action will make it possible porbably to use tRNA instead of circular nucleic acids for endonuclease admixtures estimation in enzyme preparations.