Demethylation of the gene expressing a yolk protein precursor in quail

Japanese quail produce more egg-yolk protein per gram of body weight than chickens do. Because cytosine phosphoguanine (CpG) methylation has been reported to correlate inversely with gene activity, the demethylation of cytosine residues in the quail vitellogenin 2 (Vg2) gene was assessed as a possible contributing factor. Vitellogenin genes are normally transcribed in the liver of female Japanese quail that are sexually mature. Changes in the methylation pattern of CCGG sites of the quail Vg2 gene were studied using the methyl-sensitive restriction endonucleases Msp I and Hpa II. At least five CCGG sites were detected within the transcribed region of the Vg2 gene; one or two of these sites were hypomethylated in all tissues examined in male and female quail, two sites underwent transcription-dependent demethylation in the liver of laying females and also in the liver of males following estradiol injection (64 mumol estradiol/mL of absolute ethanol at .25 mL/100 g of BW), and another site was demethylated in the liver of estradiol-injected males but not in laying females. Two further sites present within the 5' flanking sequences or 5' structural region underwent transcription-dependent demethylation in the liver of laying females and of estradiol-treated males; one or both of these sites appears to be demethylated in the oviduct. Another site, within the 3' flanking sequences or the 3' structural gene region, underwent estradiol-dependent demethylation in the liver and the oviduct. The expression-linked demethylation of the CCGG sites in the Vg2 gene of quail appears to be similar to that reported for the chicken VTGII gene. The results of this study did not explain the enhanced production of yolk protein in Japanese quail, but did suggest the presence of a fundamental, conserved mechanism that must play an important role in the expression of the avian Vg gene.