Expression-linked demethylation of 5-methylcytosines in the chicken vitellogenin gene region

We have studied the methylation status of the estradiol-controlled chicken vitellogenin (Vtg) gene, which is expressed in the liver. A 30-kb region was investigated, containing 17 HpaII and 18 HhaI sites, of which 21 are in the 22-kb gene. Of these 21 sites, 9 were found to be demethylated in laying-hen liver relative to immature chicken liver. Outside the transcribed region, only one site was found to be relatively undermethylated in laying-hen liver. This site, at 0.6 kb in front of the gene, is, as shown earlier, also demethylated in rooster or immature chicken liver upon primary hormone stimulation, as well as in the non-expressing estradiol target organ oviduct. In this respect, this site sharply contrasts with those in the transcribed region, which appear to become demethylated only upon prolonged transcription of the gene.