Free 3'-OH group of the terminal adenosine of the tRNA molecule is essential for the synthesis in vitro of guanosine tetraphosphate and pentaphosphate in a ribosomal system from Escherichia coli.

tRNAPhe from yeast modified at the 3'-terminal adenosine residue or missing part of its CpCpA end was investigated for its ability to participate in the synthesis of guanosine tetraphosphate and pentaphosphate using Escherichia coli ribosomes. Only tRNA containing complete CpCpA end and an intact ribose residue in the terminal adenosine was active in this process. The absence of the 2'-hydroxyl group of the terminal adenosine does not influence the reaction, but the 3'-hydroxyl group is essential and cannot be replaced by an amino group or a hydrogen atom. tRNA in which the 3'-terminal adenosine is replaced by formycin is active in the induction of guanosine tetraphosphate and pentaphosphate synthesis, but the reaction proceeds relatively slowly.