Abstract
Genes coding for unique or tissue-specific (differentiated) functions in the liver are induced at different times during development. It has generally been felt that transcriptional control represents the dominant mechanism for regulating expression of these genes. We have determined the relative transcription rates and mRNA steady-state levels for a series of genes specifically or preferentially expressed in rat liver and find examples of transcriptional control (albumin, alpha-fetoprotein, alpha 1-antitrypsin, tyrosine aminotransferase, transferrin, and cytochrome P450, TF-1) and post-transcriptional control (alpha 1-acid glycoprotein, apolipoproteins A-1 and E, malic enzyme, and ATP citrate lyase), as well as "mixed" regulation (ligandin and cytochrome P450, R17). Examples have been identified in which the predominant mode for regulating expression of preferentially expressed genes changes from transcriptional to post-transcriptional at different stages of liver development and some members of multigene families (cytochrome P450s and apolipoprotein genes) also show independent and sometimes contrasting modes of regulation. Therefore, it appears that regulation of specific gene expression in the liver is a dynamic process, far more complex than heretofore suspected, and a much greater contribution of post-transcriptional regulation accounts for changes in expression of genes representing major functions of the liver.