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In Situ Permeable Reactive Barriers for Groundwater Contamination
In situ permeable reactive barriers (PRBs) consist of zones of reactive material, such as granular iron or other typically reduced metal, lime, electron donor-releasing compounds, or electron acceptor-releasing compounds, installed in the path of a plume of contaminated groundwater. As the groundwater flows through this zone, contaminants are degraded to innocuous components through chemical and/or biological reactions, adsorbed, or chemically altered so that they form insoluble precipitates. This article represents a summary review of representative literature on permeable reactive barrier technology. It consists of a description of the technology, a list of treatable contaminants, the processes necessary for its implementation, considerations for conducting performance monitoring, a discussion of the positive and negative attributes and costs of the technology, and lessons learned during recent applications. Where conditions are favorable and time factors are appropriate, this technology appears promising. The main characteristic in its favor is the lack of the need to operate pumps or treatment vessels, thereby saving operation and maintenance costs and allowing the economic value of property to be restored during remediation. Its reliance on natural advec-tive processes to move contaminants through the treatment zone, resulting in long treatment time frames, can be a disadvantage under some circumstances. There are also uncertainties about the long-term effectiveness of the reactive media. Regulators need to continue the trend toward being more receptive of this technology, as well as other innovative technologies, so that it can be improved. This receptiveness will benefit all stakeholders involved.
Journal Article
Termination Efficiency at Rho-Dependent Terminators Depends on Kinetic Coupling Between RNA Polymerase and Rho
Rho-dependent terminators constitute one of two major classes of terminators in Escherichia coli. Termination at these sites requires the concerted action of RNA polymerase and rho protein. We present evidence that the efficiency of termination at these sites is governed by kinetic coupling of the rate of transcription of RNA polymerase and the rate of action of rho protein. Termination experiments in vitro indicate that termination efficiency at a rho-dependent terminator is an inverse function of the rate of elongation of RNA polymerase, and each of the mutant phenotypes can be accounted for by the altered rate of elongation of the mutant RNA polymerase. Experiments in vivo show that fast-moving mutant RNA polymerases are termination deficient, while slow-moving mutant RNA polymerases are termination proficient and can suppress the termination deficiency of a slow-acting mutant rho protein. Because of the close coupling of rho action with RNA polymerase, small changes in the elongation rate of RNA polymerase can have very large effects on termination efficiency, providing the cell with a powerful way to modulate termination at rho-dependent terminators.
Journal Article
Crystal structure of the RNA-binding domain from transcription termination factor rho
Transcription termination factor rho is an ATP-dependent hexameric helicase found in most eubacterial species. The
Escherichia coli
rho monomer consists of two domains, an RNA-binding domain (residues 1–130) and an ATPase domain (residues 131–419). The ATPase domain is homologous to the β subunit of F1-ATPase. Here, we report that the crystal structure of the RNA-binding domain of rho (rho130) at 1.55 Å confirms that rho130 contains the oligosaccharide/oligonucleotide-binding (OB) fold, a five stranded β-barrel. The β-barrel of rho!30 is also surprisingly similiar to the N-terminal β-barrel of Fl ATPase, extending the applicability of Fl ATPase as a structural model for hexameric rho.
Journal Article
NusG is Required to Overcome a Kinetic Limitation to Rho Function at an Intragenic Terminator
Rho-dependent transcription termination at certain terminators in Escherichia coli also depends on the presence of NusG [Sullivan, S. L. \\& Gottesman, M. E. (1992) Cell 68, 989-994]. We have found that termination at the first intragenic terminator in lacZ (tiZ1) is strongly dependent on NusG when transcription is done in vitro with the concentrations of NTPs found in vivo. With a lower level of NTPs, and consequently a slower rate of RNA-chain growth, Rho causes some termination by itself that is enhanced with NusG. These results suggest that NusG serves to overcome a kinetic limitation of Rho to function at certain terminators. At a second intragenic terminator within the lacZ reading frame (tiZ2) the efficiency of Rho-mediated termination was unaffected by either NusG or by RNA polymerase elongation kinetics. Thus, using purified components and intracellular levels of NTPs, we have confirmed the in vivo finding that certain Rho-dependent terminators also depend on NusG, whereas others do not.
Journal Article
An RNA-Dependent Nucleoside Triphosphate Phosphohydrolase (ATPase) Associated with Rho Termination Factor
Highly purified rho termination factor from Escherichia coli catalyzes an RNA-dependent hydrolysis of ribonucleoside triphosphates to nucleoside diphosphates and inorganic phosphate. In the presence of poly(C), a specific activity of 100 μ mole of ATP hydrolyzed per min/mg has been measured. The phosphohydrolase activity appears to be associated with the protein responsible for termination of RNA synthesis, but a functional relationship between the two activities is not yet evident. Hydrolysis of nucleoside triphosphates occurs in the absence of termination and without any extensive degradation of RNA.
Journal Article
