Post-transcriptional regulation of glutamyl-prolyl-tRNA synthetase in rat salivary gland.

Treatment of rats with isoproterenol resulted in elevated levels of prolyl- and glutamyl-tRNA synthetase activities in the parotid and submandibular glands. This increase in enzyme activity was accompanied by an increase in the bi-functional glutamyl-prolyl-tRNA synthetase and of a low molecular weight form of prolyl-tRNA synthetase. Isoproterenol also induced the synthesis of proline-rich glycoproteins in the parotid and submandibular glands. Withdrawal from the drug was accompanied by a decline in prolyl- and glutamyl-tRNA synthetase activities and by a decline in the levels of proline-rich glycoproteins in the salivary gland. During the time course of isoproterenol treatment, little change in the levels of mRNA encoding the bi-functional glutamyl-prolyl-tRNA synthetase was detected by Northern blot analysis. These results indicate that the synthesis of glutamyl-prolyl-tRNA synthetase is regulated at a post-transcriptional step and that the synthesis of this bifunctional protein may be linked to the utilization of proline and glutamic acid in protein synthesis.