Continuous silicon carbide fiber of high-tensile strength (about 350 Kg/mm2) was synthesized by means of the heat-treatment of organosilicon polymer obtained from dodecamethylcyclohexasilane. X-ray analysis and high voltage electron microscopic observations revealed that the obtained continuous fiber is an ultrafine grain structure of β-silicon carbide.

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The chromosomal features that influence retroviral integration site selection are not well understood. Here, we report the mapping of 226 avian sarcoma virus (ASV) integration sites in the human genome. The results show that the sites are distributed over all chromosomes, and no global bias for integration site selection was detected. However, RNA polymerase II transcription units (protein-encoding genes) appear to be favored targets of ASV integration. The integration frequency within genes is similar to that previously described for murine leukemia virus but distinct from the higher frequency observed with human immunodeficiency virus type 1. We found no evidence for preferred ASV integration sites over the length of genes and immediate flanking regions. Microarray analysis of uninfected HeLa cells revealed that the expression levels of ASV target genes were similar to the median level for all genes represented in the array. Although expressed genes were targets for integration, we found no preference for integration into highly expressed genes. Our results provide a more detailed description of the chromosomal features that may influence ASV integration and support the idea that distinct, virus-specific mechanisms mediate integration site selection. Such differences may be relevant to viral pathogenesis and provide utility in retroviral vector design.Retroviral DNA integration is catalyzed by a viral enzyme, integrase (IN), which nicks the two ends of linear viral DNA and splices them into a site in the host DNA (9, 10). This highly orchestrated reaction produces DNA sequence signatures at the virus-host junctions: the loss of usually 2 bp at the ends of the linear viral DNA and duplication of several base pairs of host DNA at the integration site. However, no gross rearrangements or deletions of either the viral or host DNAs are incurred. The integration reaction can be reproduced in vitro using purified IN and viral and target DNA model substrates. Despite the precision of the integration reaction, there are no strict host DNA sequence requirements. Nevertheless, numerous studies indicate that integration site selection is not likely to be entirely random. For example, various features of host chromosomes have been implicated in influencing integration site selection, including primary DNA sequence (7,14,18), DNA structure (16,20,24), nucleosome structure (27-31), chromatin structure (32,36,37,40), and transcriptional activity (23,33,34,41,43). In addition to the passive influences of chromosomal structure, it has been suggested that retroviral integration could be actively targeted by tethering to specific, chromatin-bound host factors (5). Lastly, the IN proteins from different retroviruses produce unique in vitro integration patterns in naked DNA targets (18). These intriguing but disparate observations have not yet led to a unifying model, and the mechanisms that govern integration site selection in vivo remain obscure.In an infected cell, retroviral DNA is organized in a preintegration complex that in...

Polycarbosilane as the precursor of continuous SiC fibre was synthesized by thermal decomposition of polydimethylsilane. The structure of the polycarbosilane is concluded to be similar to that of polysilapropylene by the measurements of i.r. spectra, NM R spectra and chemical analyses. Its formation mechanisms are initially the formation ofcarbosilane by thermal decomposition of polydimethylsilane and then the increase in molecular weight by dehydrogenation-condensation of the carbosilane. Molecular structure and molecular weight distribution of the polycarbosilane depend on the reaction temperature.