Showing papers on "Nucleolar chromatin published in 1985"

Structural organization of an active, chromosomal nucleolar organizer region (NOR) identified by light microscopy, and subsequent TEM and STEM electron microscopy.

Abstract: The three-dimensional arrangement of the chromatin components within the nucleolar organizer regions (NORs) from living oocyte nuclei was investigated. As a suitable cell system we chose vitellogenic oocytes of the orthopteran insect Acheta. This cell type is particularly attractive for analysis of nucleolar chromatin, since structural and functional aspects of NORs during early oogenic stages (including the association of NORs with amplified rDNA copies) are particularly well known (Lima-de-Faria 1974). In the course of the present study we first identified putative chromosomal NORs in isolated nuclei of mid-diplotene oocytes according to morphological characteristics using differential interference contrast (DIC) or phase contrast light microscopy. The presence of actively transcribing chromosomal NORs during this late stage of Acheta oogenesis obviously had been overlooked by previous investigators, probably due to difficulties of chromosome visualization. For a more detailed ultrastructural analysis, NORs were gently sedimented from opened nuclei and processed for sectioning using a modified “end-embedding” procedure (Mott and Callan 1975; Spring and Franke 1981). A small number of thick and thin sections could be made from individual NORs. Sections were analyzed by light microscopy, conventional transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). Whereas the structural connection of NORs to the chromosome axis and also the general arrangement of active nucleolar genes within the NOR complex could be seen with TEM, the visualization of individual nucleolar genes and the organization of transcription complexes was only possible using bright field STEM of thick sections at low temperature.

Regulation of uterine nucleolar RNA synthesis by estrogens.

Abstract: Administration of estradiol to ovariectomized mature rats results in a biphasic early (4 h) and late (24 h) increase in transcriptional activity of isolated uterine nucleoli. The increased rate of nucleolar RNA synthesis is dependent upon the dose of estradiol over the range of 0.1 to 1 �ig/anima!, and exhibits hormone specificity. Administration of cycloheximide, an inhibitor of protein synthesis, prior to the administration of hormone or during the early phase of estrogen action ( 8 h) of estrogen action is without effect on the estrogen-induced increase in transcriptional activity of isolated uterine nucleoli. This suggests that the longer term maintenance of the hormone-stimulated increase in nucleolar RNA synthesis is independent of continuous protein synthesis. Results indicate that the estrogen-induced accumulation and subsequent decline in uterine nuclear estradiol receptor levels is unaffected by cycloheximide treatment. Together, these results indicate the presence of the receptor-hormone complex in the nucleus is not solely responsible for the increased transcriptional activity of uterine nucleoli following in vivo hormone treatment. The early activation of uterine nucleolar RNA synthesis by estrogen seems to result from the synthesis of a short-lived protein(s) that modified RNA polymerase I and/or the nucleolar chromatin template.