The avian apoprotein II very low density lipoprotein gene. Methylation patterns of 5' and 3' flanking regions during development and following induction by estrogen.

Apoprotein I1 very low density lipoprotein (apo-VLDLII) mRNA is one of the major abundant mRNA species present in hen liver.It specifies a major component of the very low density lipoprotein fraction of egg yolk.Expression of the apoVLDLI1 gene normally begins in the female chick at the onset of vitellogenesis.However, apoVLDLII synthesis can be induced in roosters or embryos by administration of pharmacological doses of estrogenic steroids.We have examined regions flanking the apoVLDLII gene for the presence of other expressed sequences and for alterations in methylation patterns that correlate with its state of activity.We find no evidence of other estrogen-dependent or constitutively expressed genes within a 25-kilobase pair region spanning the apoVLDLI1 gene.Inverted-repeat sequences were found in the vicinity of the 5' end of the gene that hybridized with polyadenylated RNA from several tissues.However, we could detect no specific transcript, polyadenylated or otherwise, that originated from these sequences.Methylation patterns of 5' and 3' flanking regions of the gene were investigated at different stages of embryogenesis, as well as in both normal and estrogen-treated roosters.These studies revealed that demethylation of an Msp I site, 2.6 kilobase pairs upstream from the 5' end of the gene, and an Xho I site, 1.6 kilobase pairs downstream from the 3' end, occurs between day 7 and 9 of embryogenesis.Hybridization analyses indicate that it is during this period that the embryo also acquires competence to express the apoVLDL11 gene.As far as we have been able to determine, the adult methylation pattern of regions flanking the gene is established in embryonic liver by day 9 and remains unchanged following activation of the gene in estrogen-treated roosters.Regulation of genes encoding the abundant mRNA class of avian liver differs strikingly from that observed in mammals.Approximately 30% of the protein synthesized in the liver of the hen is destined for deposition in the egg yolk (1).These proteins are encoded by a group of estrogen-responsive genes that exhibit varying degrees of dependence on the hormone and a wide range of rates of expression (2-8).Two of the proteins: vitellogenin, the major component of the high density lipoprotein fraction of yolk, and apoVLDLI1,' a major