Crystallization sequence of an A1N-Y₂O₃-SiO₂ glass-ceramic

The use of Si3N4, alloys for high temperature, high stress structural applications has prompted numerous studies of the oxynitride glasses which exist as intergranular phases in their microstructures. Oxynitride glasses have been investigated recently in their bulk form in order to understand their crystallization behavior for subsequent Si3N4 applications and to investigate their worth as glass-ceramic precursors. This research investigates the crystallization sequence of a glass having a normalized composition of Y26Si30Al11 ON11 and lying in the A1N-Y2O3-SiO2 section of the Y-Si-Al-O-N system. Such glasses exist as intergranular phases in the technologically important Y2O3/Al2O3-fluxed Si3N4 alloys.</jats:p

Tem studies of partially crystallized Y-Si-Al-O-N glasses

Although Si3N4 has many attractive refractory properties some limitations occur due to intergranular oxynitride glass phases that result from liquid phase sintering with oxide additions. Hence it is of fundamental and technical importance that a more detailed understanding of such glasses and possible crystallization behavior is obtained.This research is being done on a series of compositions of Y-Si-Al-O-N which can be made as glasses and subsequently heat treated to promote crystallization. Here the results of crystallization (of glass Y.26Si.30Al.11ON.11) leading to Y2Si2O7 crystals of different forms is described. Specimens for electron microscopic observations were prepared by ion milling followed by a thin layer of carbon and examined by conventional TEM, CBED, EDXS and at atomic resolution in the Berkeley ARM.</jats:p

A new class of non-coding RNAs associated with 3\u2019 untranslated regions of mRNAs

The importance of non-coding RNAs (ncRNAs) in controlling gene expression is becoming increasingly evident. However, except for some well characterized examples, such as miRNAs, Xist and Air, the function of most non-coding transcripts is still to be determined. Moreover, while small regulatory RNAs can be relatively easily classified on the basis of their length, secondary structure, and biochemical pathway, the classification of long \u201cmRNA-like\u201d ncRNAs has been problematic. Here we identify a large class of non-coding transcripts that originate within the 3\u2019UTR of at least one third of all genes in the mouse genome. We have several lines of evidence from genome-wide bioinformatic analyses (EST coverage, CAGE data, chromatin state maps of active promoters) and from invitro studies (strand-specific RT-PCR, 5\u2019RACE, Northern blot) showing that these 3\u2019UTR-associated ncRNAs (uaRNAs) can be either linked or transcribed separately to the upstream protein-coding sequences. In addition, expression profiles obtained by custom-designed microarrays on three different developmental systems (myoblast differentiation, male gonadal ridge formation, embryonic stem cell differentiation) showed that uaRNA expression is highly regulated and tissue-specific, and might be either concordant or discordant with respect to the upstream coding region depending on the cell type and on the developmental stage. This observation is confirmed by in-situ hybridization experiments, which evidenced that uaRNA and the associated coding transcript might have different subcellular locations. Our results highlight a further level of complexity at 3\u2019UTRs, suggesting the presence of new regulatory mechanisms that control gene expression during embryonic development. Our data have also important implications for the design of in-situ hybridization and microarray probes as well as for the interpretation of gene expression dat

Regulated independent expression of 3' untranslated regions in mammals

Eukaryotic mRNA localization, translation and stability are regulated by 3' untranslated regions (3'UTRs). 3'UTRs control mRNA expression in cis, via regulatory elements that are recognized by trans-acting factors, including RNA-binding proteins and microRNAs. However, a recent analysis of transcription start sites in mouse suggested that 3'UTRs may also be the source of independent transcripts, but there have been no in vivo analyses to confirm this suggestion. Here we report that a large number of 3'UTRs are not only linked to but are also expressed separately from their associated protein-coding sequences. We identify ~14,000 mouse genes exhibiting independent expression of 3'UTRs, of which 1,385 were supported by two distinct experimental approaches, and ~100 confirmed by microarray expression profiles. In situ hybridization demonstrated that 3'UTR-associated transcripts (uaRNA) can be expressed as part of mRNAs or discordantly in a developmentally-regulated and cell-specific manner, in some cases localizing to the nucleus. We also provide evidence that chromatin remodelling and transcription factor networks are involved in regulating uaRNA expression. These observations suggest that 3'UTRs not only function in cis to regulate protein expression but also in trans as noncoding RNAs, a conclusion supported by genetic studies dating back over a decade. Our findings prompt a re-evaluation of 3'UTR biology and the complexity of genome architecture in mammals