Stereochemistry of internucleotidic bond formation by tRNA nucleotidyltransferase from bakers' yeast

Abstract

Isomer A of adenosine 5''-O-(1-thiotriphosphate) (ATP.alpha.S) is a substrate for tRNA nucleotidyltransferase [EC 2.7.7.25] from baker''s yeast, whereas isomer B is a competitive inhibitor. The tRNA resulting from this reaction has a phosphorothioate instead of a phosphate diester linkage at the last internucleotidic linkage between cytidine and adenosine. On limited digestion of this tRNA with RNase A, cytidine 2'',3''-cyclic phosphorothioate was isolated which was deaminated to uridine 2'',3''-cyclic phosphorothioate. It was shown that this compound is the endo isomer and that, therefore, the phosphorothioate diester bond in the tRNA must have had the R configuration. This result indicates that no racemization during the condensation of ATP.alpha.S, isomer A, onto the tRNA had occurred. whether inversion or retention of configuration had taken place awaits elucidation of the absolute configuration of isomer A of ATP.alpha.S.