Chromatin structural transitions and the phenomenon of vitellogenin gene memory in chickens

We have previously shown that the steroid hormone-mediated transcriptional activation of the chicken vitellogenin II gene (VTGII) in the liver is accompanied by a series of chromatin structural changes, including the formation of two sets of 5'-proximal nuclease-hypersensitive sites and the demethylation of a single 5'-flanking MspI site which lies within a region of DNA that recently has been shown by Jost and co-workers to specifically bind the estrogen receptor complex in vitro. To assay the stability and possible functional significance of these induced structural changes, we transiently activated the VTGII gene during embryonic development and then allowed the chickens to hatch and grow for various periods of time before analyzing their livers. By 7 weeks posthatching all of the induced 5'-flanking hypersensitive sites had decayed. Moreover, the loss of these sites occurred without consequence to the "memory effect," that is, these structural features did not need to be present in hormone withdrawn birds to allow this gene to be activated more rapidly in response to a secondary presentation of estradiol. Although the demethylation was more stable, it also appeared not to be the basis of the memory phenomenon. The birds that still exhibited memory after 25 weeks of hormone withdrawal were not more extensively demethylated within the receptor-binding site than were the birds which failed to show memory at this age. A similar uncoupling of these two parameters was also observed when embryos were first injected with submaximal doses of estradiol and then assayed 1 week after hatching; the chickens which acquired memory were not demethylated to any greater extent than those which did not acquire memory. Other parameters that may be relevant to memory are discussed.