Transcription of complementary repeat sequences in amphibian oocytes

Long non-coding RNAs: roles in cellular stress responses and epigenetic mechanisms regulating chromatin

Most of the genome is transcribed into RNA but only 2% of the sequence codes for proteins. Non-coding RNA transcripts include a very large number of long noncoding RNAs (lncRNAs). A growing number of identified lncRNAs operate in cellular stress responses, for example in response to hypoxia, genotoxic stress, and oxidative stress. Additionally, lncRNA plays important roles in epigenetic mechanisms operating at chromatin and in maintaining chromatin architecture. Here, we address three lncRNA topics that have had significant recent advances. The first is an emerging role for many lncRNAs in cellular stress responses. The second is the development of high throughput screening assays to develop causal relationships between lncRNAs across the genome with cellular functions. Finally, we turn to recent advances in understanding the role of lncRNAs in regulating chromatin architecture and epigenetics, advances that build on some of the earliest work linking RNA to chromatin architecture.

The ribonucleoprotein network of the nucleus: a historical perspective

There is a long experimental history supporting the principle that RNA is essential for normal nuclear and chromatin architecture. Most of the genome is transcribed into RNA but only 2% of the sequence codes for proteins. In the nucleus, most non-coding RNA, packaged in proteins, is bound into structures including chromatin and a non-chromatin scaffolding, the nuclear matrix, which was first observed by electron microscopy. Removing nuclear RNA or inhibiting its transcription causes the condensation of chromatin, showing the importance of RNA in spatially and functionally organizing the genome. Today, powerful techniques for the molecular characterization of RNA and for mapping its spatial organization in the nucleus have provided molecular detail to these principles.

Transcriptional activity of constitutive heterochromatin in the mammal Microtus agrestis (Rodentia, Cricetidae)

Two cytological approaches were applied to the analysis of genetic activity in euchromatin and heterochromatin in the European field vole Microtus agrestis. The first is based on the transcriptional activity of prematurely condensed chromosomes, the second on in situ hybridization of labeled cellular RNA to metaphase chromosomes. The results show that in cultivated fibroblasts, the transcriptional activity of constitutive heterochromatin is in the same order of magnitude as that of euchromatin; its chromosomal pattern is nonrandom. It is concluded that--as has already been shown for insects and amphibians--also in mammals, constitutive heterochromatin is transcribed in its decondensed state but is genetically inactive when forming chromocenters at interphase.

Identification of a 60-kDa phosphoprotein that binds stored messenger RNA of Xenopus oocytes

Rapidly labelled, polyadenylated RNA is contained in three distinct fractions isolated from homogenized amphibian oocytes: (a) in ribonucleoprotein particles that are associated with a fibrillar matrix, the complexes sedimenting at greater than 1500S; (b) in ribonucleoprotein particles that sediment at 20-120S and have the characteristics of stored (maternal) messenger ribonucleoprotein (mRNP) and (c) in polyribosomes that sediment at 120-360S. We have compared the RNA and protein components of the first two of these RNP fractions. The polyadenylated RNA extracted from the two RNP fractions differs in that the RNA from fibril-associated RNP contains a much higher content of repeat sequences than does the RNA from mRNP. In other words, the RNA from fibril-associated RNP is largely unprocessed and may constitute a premessenger state, which for convenience is referred to as premessenger RNP (pre-mRNP). RNA-binding experiments demonstrate that the polypeptide most tightly bound in pre-mRNP is a 54-kDa component (p54), whereas the polypeptide most tightly bound in mRNP is a 60-kDa component (p60). Antibodies raised against p60 are used to show that this polypeptide is a common major component of pre-mRNP and mRNP and that it is also located in oocyte nuclei. However the state of p60 is modified between the premessenger and stored message levels: the polypeptide in mRNP is heavily phosphorylated whereas the equivalent polypeptide in pre-mRNP is completely unphosphorylated. The relative roles of the presence of repeat sequences and phosphorylation of mRNA-associated protein in blocking translation are discussed.