Rat alpha 1-fetoprotein messenger RNA: 5'-end sequence and glucocorticoid-suppressed liver transcription in an improved nuclear run-off assay.

Cloned cDNA fragments spanning nearly the entire coding regions of rat AFP and albumin genes were used in liver nuclear run-off assays. Under standard assay conditions, transcription signals detected with 5' probes were systematically stronger than with 3' probes. Heparin eliminated this phenomenon, which suggests that nuclear run-off assays are subject to in vitro reinitiation occurring preferentially in promoter gene regions. Transcription in the presence of heparin indicates that very few polymerases are engaged on the AFP gene in adult rat liver. Dexamethasone treatment of developing rat liver results in the loss of transcribing polymerases from all regions of the AFP gene. Albumin gene transcription is unaffected. Inhibition of liver protein synthesis with cycloheximide does not modify the AFP gene suppressive action of dexamethasone. Glucocorticoid hormone receptors may thus directly interact with the AFP locus, blocking polymerase initiation. We also report the sequence analysis of rat AFP mRNA, which reveals the existence of two potential initiation codons on this molecule.