Showing papers on "Nucleolar chromatin published in 1982"

Structure of the active nucleolar chromatin of Xenopus laevis oocytes.

Abstract: Active nucleolar chromatin of Xenopus laevis oocytes was prepared for electron microscopy by a step gradient method, which separates the chromatin from proteins and other constituents which might unspecifically bind at low ionic strength. Between putative RNA polymerases and within the non-transcribed spacer region, the chromatin appears as smooth, thin filaments, indistinguishable from pure DNA adsorbed to the same specimen. These filaments are found under all conditions tested, even in the presence of 100 mM NaCl. On the other hand, bulk rat liver chromatin, which was co-prepared with the active nucleolar chromatin, shows nucleosomes containing fibers, which condense into supranucleosomal structures with increasing ionic strength. Since the appearance and the behaviour of active nucleolar chromatin at different ionic strength and pH resembles that of pure DNA, but not that of any known type of chromatin, it is suggested that, except for the transcription apparatus, few macromolecular constituents are associated with ribosomal DNA during transcription.

DNA Dependence and Inhibition by Novobiocin and Coumermycin of the Nucleolar Adenosine Triphosphatase (ATPase) of Human Fibroblasts

Abstract: We have recently demonstrated by electron microscopic cytochemical methods that unfixed human fibroblasts exhibit intense Mg2� dependent adenosine tniphosphatase (nATPase) activity in circumscribed areas of the cell nucleoli. The nATPase was specific for ATP and dATP and was inhibited by other ribonucleoside tniphosphates. Its intranucleolar localization relative to nucleolar chromatin, and segregation into nucleolar zones after actinomycin treatment of the cells, suggested that the reaction took place in fibnillar centers. This ATPase has now been further characterized by electron microscopic cytochemistry. It was determined that short fixation permitted retention of most of the ATPase activity, and that the enzyme was active at high ionic strength (up to 400 mM KCI), but that the enzyme activity was very sensitive to elevated temperatures. DNA dependence of the enzyme was shown by

Chromatin structure of Xenopus rDNA transcription termination sites. Evidence for a two-step process of transcription termination.

Abstract: The ultrastructure of Xenopus laevis (X.l.) 40S pre-rRNA transcription termination sites was investigated by electron microscopy. Amplified nucleolar chromatin was rapidly dispersed and processed for chromatin spread preparations. This type of preparation revealed that many rDNA termination sites appeared as 50 nm long segments of chromatin axis covered by a complex of three closely spaced RNA polymerase particles. Particle (Pt1) was characterized by the association with the terminal full-length pre-rRNP fibril; particles (Pt2) and (Pt3) are located downstream from (Pt1) and appear to be devoid of transcript fibrils. This particular structural arrangement as well as sequence homology analyses of 3′ adjacent spacer rDNA segments indicate that transcript release and dissociation of polymerase particles are not necessarily coupled and termination of X.l. rDNA transcription may occur as a two step process. The structural data are correlated with homologies of DNA sequences at Xenopus rDNA transcription termination regions and are discussed with respect to sequence data of 3′ termination sites of rRNA genes from other species.

Evidence for the transcription of physiologically inactive rat-liver nucleolar chromatin byEscherichia coli RNA polymerase

Abstract: Rat-liver nucleoli (10-15 pg DNA) were digested with either 0.6 or 3 units of DNase I for various times (up to 1 h). RNA synthesis was then measured in the absence or presence ol 3 units ofEscherichia coli RNA polymerase. It was found that the nucleolar chromatin supporting the endogenous engaged RNA polymerase I transcription was compl-etely destroyed in 3 min with either concentration of DNase I. The nucleolar chromatin template transcribed byE. coli RNA polymerase retained 50% of its original capacity even 60 min alter 3 units of DNase I digestion. When hybridization experiments were conducted, it was found that the DNAs derived from both levels of DNase-Idigested nucleoli were incapable of forming hybrids with the labelled nucleolar RNA synthesized by the engaged RNA polymerase I from the untreated nucleoli. Since the engaged RNA polymerase I transcribes only the physiologically active genes of the nucleolar chromatin, and the RNA transcripts represent active gene product, these data suggest that DNase I digestion has completely destroyed the active genes of the nucleolar chromatin, andE. coli RNA polymerase is able to transcribe the inactive nucleolar chromatin template.