Enhanced sedimentation coefficient

Eukaryotic ribosomes contain four pieces of RNA (Tables 5-4 and 29-1), which are usually designated by their sedimentation coefficients. Tire 18S, 5.8S, and 28S RNAs are encoded as single transcriptional units with spacers separating the sequences that encode the mature RNAs. A typical animal cell contains several hundred copies of this transcriptional unit, all located in the DNA in the nucleolus (Fig. 28-15), and each having its own set of promoter sequences, enhancers, and transcription factors.47 520 522 The promoter sequences vary substantially among different species.523 The primary transcripts from these units are the sole product of RNA polymerase I. 

Cell-free system. The first experiment with cell-free systems containing labeled nuclei and nonlabeled cytoplasm (Schneider, 1959 Scholtissek and Potter, 1960) indicated the removal of newly formed RNA from the nuclei during the incubation. The newly synthesized RNA was found in the incubation medium in particles with different sedimentation coefficients. These results have been confirmed by others (Samarina and Zbarsky, 1964 Ishikawa et al., 1969 Lukanidin, 1969). Sometimes ATP and other donors of energy enhance the loss of RNA from nuclei (Ishikawa et al., 1969). However, this process seems not to differ from the simple extraction of D-RNP from nuclei and thus may not be a model of transport. Lukanidin (1969) analyzed the particles in a CsCl density gradient and found that almost all labeled material leaving the nucleus has a buoyant density of 1.40 g/cm and does not interact with the cytoplasmic ribosomes which band at p = 1.55 to 1.58 g/cm. Only a small amount of labeled material banded in the intermediate zone, but it had a base composition similar to rRNA and thus may represent the ribosomal RNA precursors. The addition to the system of a variety of factors necessary for protein synthesis did not influence the results. 

Sorption of pharmaceuticals onto the surface of particulate matter or their distribution between two phases (water and either sludge, sediment or soil) depends on many factors, the most important being liquid phase pH and redox potential, the stereochemical structure and chemical nature of both the pharmaceutical compound and the sorbent, the lipophilicity of the sorbed molecules (excellent sorption at log Kov > 4, low sorption at log < 2.4), the sludge-water distribution coefficient Kd Kd > 2 L g SS good sorption, < 0.3 L g SS low sorption), the extent of neutral and ioiuc species present in the wastewater and the characteristics of the suspended particles. Moreover, the presence of humic and fulvic substances may alter the surface properties of the sludge, as well as the number of sites available for sorption and reactions, thereby enhancing or suppressing sorption of PhCs [38, 55, 61]. 

Spiral-plate exchangers provide high surface densities. The induced secondary flow results in enhanced heat-transfer coefficients, especially for viscous fluids, and reduced sedimentation-type fouling. Spiral-plate exchangers can be easily opened for cleaning by high-velocity water jets. 

Enhanced chemical transport in the upper sediment layer by the process called bioturbation is termed biodiffusion. It moves both the particle bound and soluble chemical fractions due primarily to the activities of macroinvertabrates and is not dependent on a chemical concentration gradient. The parameters Ep and Ed in Equations 2 and 4 are termed the particle and pore water biodiffusion coefficients respectively (L t). 

Experimental and mathematical modeling studies were performed to evaluate the potential benefits and limitations associated with the use of nonionic surfactants to enhance the microbial transformation of hexachlorobenzene (HCB) by a dechlorinating mixed culture enriched from a contaminated sediment. In general. Tween series surfactants were shown to have little impact on methanogenesis, whereas, polyoxyethylene (POE) alcohols, Triton X-100 and SDS were found to strongly inhibit methanogenesis and HCB dechlorination. Subsequent experiments conducted with Tween 80 illustrated the ability of this surfactant to enhance the solubility of HCB and to reduce the HCB-soil distribution coefficient. Model simulations demonstrated, however, that the aqueous phase mass fraction of HCB was substantially reduced in micellar solutions, which corresponded with observed reductions in HCB dechlorination. These results indicate that the impacts of surfactants on both biological activity and contaminant phase distributions should be evaluated in order to accurately assess the potential for biotransformation of hydrophobic contaminants in the presence of surfactants. 

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