The inductive effect of a human DNA sequence (HALF1) on the differentiation of a variant rat hepatoma cell (C2) is restricted to episomal forms of the molecule

HALF1, a 4.3 kb human DNA sequence, was originally identified as a double-stranded, closed-circular DNA molecule in revertants from a dedifferentiated rat hepatoma cell (C2) transfected with human liver DNA. Here we report its specific properties in inducing the transition to the hepatic phenotype. (i) In vitro recircularized HALF1 induces reversion after a minimum time lag of 7 days post-transfection. (ii) After induction, the presence of HALF1 is not required for maintaining the induced hepatic state. (iii) HALF1 is detected as a sequence integrated in high molecular mass DNA of human liver. (iv) HALF1 monomer or dimer plasmid constructs do not induce reversion when integrated into the genome of transfectants. (v) Short ubiquitous RNA transcripts (approximately 400 bases) are detected with specific HALF1 probes. These results indicate that the reversion process is linked to the presence of HALF1 extrachromosomal molecules.