INVOLVEMENT OF PROTEIN-KINASE-C IN THE CONTROL OF TRANSFER-RNA MODIFICATION WITH QUEUINE IN HELA-CELLS

The eukaryotic tRNA:guanine transglycosylase (TGT) catalyses the base- for-base exchange of guanine for queuine (the q-base)-a nutrition fact or for eukaryotes-at position 34 of the anticodon of tRNAS(GUN) (where 'N' represents one of the four canonical tRNA nucleosides), yielding the modified tRNA nucleoside queuosine (Q). This unique tRNA modificat ion process was investigated in HeLa cells grown under either aerobic (21% O-2) Or hypoxic conditions (7% O-2) after addition of chemically synthesized q-base to q-deficient cells, While the q-base was always i nserted into tRNA under aerobic conditions, HeLa cells lost this abili ty under hypoxic conditions, however, only when serum factors became d epleted from the culture medium, The inability to insert q into tRNA d id not result from a lack of substrate, because the q-base accumulated within these cells against the concentration gradient, suggesting the presence of an active transport system for this base in HeLa cells, T he activity of the TGT enzyme was restored after treatment of the cell s with the protein kinase C activator, TPA, even in the presence of mR NA or protein synthesis inhibitors, The results indicate that the euka ryotic tRNA modifying enzyme, TGT, is a downstream target of activated protein kinase C.