HORMONALLY INDUCED ALTERATIONS OF CHROMATIN STRUCTURE IN THE POLYADENYLATION AND TRANSCRIPTION TERMINATION REGIONS OF THE CHICKEN OVALBUMIN GENE

We have studied the chromatin structure of a 16-kb region of the chicken genome containing the 3''-terminal 2 kb of the ovalbumin pre-mRNA coding sequence and the 14-kb segment located immediately downstream from the main mRNA polyadenylation site. Using the indirect end-labelling technique, four major and two minor DNase I-hypersensitive regions were found in the oviduct chromatin, whereas they were not present in liver, kidney or erythrocyte chromatin. The first hypersensitive region (region A) was present in chromatin of oviducts from laying hen and estrogen- or progesterone-stimulated immature chicks, in which the ovalbumin gene is expressed, but not in the chromatin of ''acute withdrawn'' chicks where the gene is no longer transcribed. Region A spans 1.3 kb, from 7.2 to 8.5 kb downstream from the ovalbumin gene capsite (position +1), and encompasses the 3'' moiety of the last exon including the major polyadenylation signal and polyadenylation site located at +7546 and +7564, respectively. Region A also contains a minor polyadenylation signal present at +7294 and the corresponding polyadenylation site at +7368. Two putative termination sequences at +8445 and +8483 are also found at the 3'' extremity of region A in a 170-bp DNA segment within which 90% of the ovalbumin primary transcripts apparently terminate. Two minor hormone-independent DNase I-hypersensitive regions (a1 and a2) located at +8.6 and +8.8 kb are also specific to oviduct chromatin. Three additional oviduct-specific large DNase I-hypersensitive regions are situated at +13 kb (region B), +14 kb (region C) and +17 kb (region D). Of these only region D is hormone-dependent. Digestion of laying hen oviduct nuclei with micrococcal nuclease shows that the nucleosomal pattern typical of bulk chromatin is altered throughout the region investigated, whereas the transcriptionally inactive .beta.-globin gene, analyzed in the same digests, exhibits a ''typical'' nucleosomal repeat.