Analysis of the development of tyrosine aminotransferase in perinatal rat liver

The development of the hepatic enzyme tyrosine aminotransferase in perinatal rats was analyzed by translation of poly(A) RNA from perinatal rat liver in an in vitro protein synthesizing system. Before birth, enzyme levels are very low and aminotransferase mRNA activity is undetectable. This expression is refractory to transcriptional modulation, but sensitive to a limited extent to translational inducers. Analysis of the aminotransferase sequences by hybridization to complementary DNA revealed significantly fewer sequences in fetal rat liver than in RNA from adult rats. After birth, aminotransferase mRNA activity increases in concert with the increase in enzyme activity, and, at the same time becomes responsive to transcriptional regulation. Hybridization analysis of aminotransferase sequences present 12 h after birth reveal a substantial increase over fetal levels. Thus, prior to birth, transcription of the aminotransferase gene is repressed; derepression at birth leads to an increased rate of transcription of the gene and subsequent development of the enzyme.