On the Specificity of the Reduction of Transfer Ribonucleic Acids with Sodium Borohydride

Abstract

Sodium borohydride treatment of tRNA from brewer's yeast was studied with respect to the selectivity of the reaction and to the biological functions of the modified tRNAs. The results are discussed in relation to some controversial statements in the literature. 1 Borohydride treated tRNASer and tRNAPhe were digested with T1 ribonuclease. By analysis of the oligonucleotide patterns it was shown that dihydrouridine and the still unknown nucleoside Y were reduced. The conversion of dihydrouridine to ureidopropanol riboside was complete under our conditions. N4-Acetylcytidine (ac4C) in position 12 of tRNASer remained unchanged while it was easily reduced as part of the trinucleotide C-ac4C-G. 2 The acceptor activity of tRNA for serine, phenylalanine, valine, tyrosine and alanine was not diminished by borohydride treatment. Also Km and Vmax were unchanged when measured with pure tRNASer and tRNAPhe and purified synthetases or with unfractionated tRNATyr. 3 Acceptance of cytidine and adenosine 5′-phosphates by -C-A depleted tRNA was not impaired by borohydride reduction. 4 The binding to polynucleotide or trinucleotide ribosome complexes of pure tRNASer, tRNAPhe, tRNAVal and tRNATyr was not changed by borohydride treatment. This is interesting particulrly in the case of tRNAPhe where a modification had occurred right next to the anticodon. In transfer experiments no differences between reduced and unreduced Ser-tRNASer were detected.