Involvement of protein kinase C in the control of tRNA 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 factor for eukaryotes--at position 34 of the anticodon of tRNAsGUN (where 'N' represents one of the four canonical tRNA nucleosides), yielding the modified tRNA nucleoside queuosine (Q). This unique tRNA modification process was investigated in HeLa cells grown under either aerobic (21% O2) or hypoxic conditions (7% O2) after addition of chemically synthesized q-base to q-deficient cells. While the q-base was always inserted into tRNA under aerobic conditions, HeLa cells lost this ability under hypoxic conditions, however, only when serum factors became depleted from the culture medium. The inability to insert q into tRNA did 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. The activity of the TGT enzyme was restored after treatment of the cells with the protein kinase C activator, TPA, even in the presence of mRNA or protein synthesis inhibitors. The results indicate that the eukaryotic tRNA modifying enzyme, TGT, is a downstream target of activated protein kinase C.