Changes in hepatic levels of tyrosine aminotransferase messenger RNA during induction by hydrocortisone.

Messenger RNA specific for tyrosine aminotransferase was quantitated by microinjection into oocytes of Xenopus laevis. The heterologously translated enzyme was identified by specific immunoprecipitation and found to be identical with authentic aminotransferase by several criteria. The level of functional message present in rat liver increases during hydrocortisone induction, and this increase is directly proportional to the increased rate of synthesis of the enzyme. Kinetic analysis of the changes in tyrosine aminotransferase mRNA levels during induction and withdrawal indicates that the steroid does not affect the stability of the message, which has a half-life of approximately 1.2 h. Hydrocortisone, therefore, acts to increase the rate of synthesis of the specific messenger by stimulating either its transcription or processing to functional mRNA.