The KdpFABC complex of Escherichia coli, a high-affinity K+-uptake system, belongs to the group of P-type ATPases and is responsible for ATP-driven K+ uptake in the case of K+ limitation. Sequence alignments identified two conserved charged residues, D583 and K586, which are located at the center of transmembrane helix 5 (TM 5) of the catalytic KdpB subunit, and which are supposed to establish a dipole involved in energy coupling. Cells in which the two charges were eliminated or inverted by mutagenesis displayed a clearly slower growth rate with respect to wild-type cells under K+-limiting conditions. Purified KdpFABC complexes from several K586 mutants and a D583K:K586D double mutant showed a reduced K+-stimulated ATPase activity together with an increased resistance to orthovanadate. Upon reconstitution into liposomes, only the conservative K586R mutant was able to facilitate K+ transport, whereas the elimination of the positive charge at position 586 as well as inverting the charges at positions 583 and 586 (D583K:K586D) led to an uncoupling of ATP hydrolysis and K+ transport. Electrophysiological measurements with KdpFABC-containing proteoliposomes adsorbed to planar lipid bilayers revealed that in case of the D583K:K586D double mutant the characteristic K+-independent electrogenic step within the reaction cycle is lacking, thereby clearly arguing for an exact positioning of the dipole for coupling within the functional enzyme complex. In addition, these findings strongly suggest that the dipole residues in KdpB are not directly responsible for the characteristic electrogenic reaction step of KdpFABC, which most likely occurs within the K+-translocating KdpA subunit.
Hydrological assessment studies across vast regions of the arid world are often hindered by the inaccessibility of these areas and the paucity of data sets, as well as the high expenses and diffi culties entailed in acquiring these data sets, their unpublished nature, and their varying scales, projections, and datum. Using the Eastern Desert (ED) of Egypt (225,000 km 2) and the Sinai Peninsula (61,000 km 2) as test sites, we demonstrate practical and cost-effective integrated (geochemistry, geophysics, and modeling) solutions that utilize web-based geographic information system (GIS) (http://www.esrs .wmich.edu/webmap) technologies and take advantage of readily available global remote sensing data sets. Adopted methodologies allowed: (1) development of conceptual models for hydrogeologic settings conducive to groundwater entrapment and augmentation, including groundwater in fractured basement aquifers, groundwater impounded by dike swarms crosscutting alluvial aquifers, and groundwater residing in alluvial aquifers associated with ascending deep-seated fossil waters; (2) selection of criteria to identify and validate the preferred distribution of each of these aquifer types and usage of the selected criteria and observations from the GIS data sets to identify, test, and refi ne potential well locations; and (3) construction and calibration of hydrologic models to estimate average annual recharge over the major watersheds in the Sinai (463 × 10 6 m 3 /yr) and ED (171 × 10 6 m 3 /yr) and the average modern contributions to Nubian fossil aquifers (Sinai: 13 × 10 6 m 3 /yr), and to model the partitioning of precipitation as a function of precipitation amounts. The successful application of the integrated and cost-effective methodologies developed for the study areas should invite similar applications in arid regions elsewhere.
The centimeter-level positioning accuracy of real-time kinematic (RTK) depends on correctly resolving integer carrier-phase ambiguities. To improve the success rate of ambiguity resolution and obtain reliable positioning results, an enhanced Kalman filtering procedure has been developed. Based on a posteriori residuals of measurements and state predictions, the measurement noise variance–covariance matrix for double-differenced measurements is adaptively estimated, rather than approximated by an empirical function which uses satellite elevation angle as input. Since, in real-world situations, unexpected outliers and carrier-phase outages can degrade the filter performance, a stochastic model based on robust Kalman filtering is proposed, for which the double-differenced measurement noise variance–covariance matrix is computed empirically with a modified version of the IGG (Institute of Geodesy and Geophysics) III method in order to detect and identify outliers. The performance of the proposed method is assessed by two tests, one with simulated data and one with real data. In addition, the performance of F-ratio and W-ratio tests as proxies for the success of ambiguity fixing is investigated. Experimental results reveal that the proposed method can improve the reliability and robustness of relative kinematic positioning for simulation scenarios as well as in a real urban test.
<p>Multipath is a large systematic GNSS error source which can bias the Zenith Total Delay (ZTD) estimation of Precise Point Positioning (PPP). In this paper, we explore the magnitudes and systematics of the errors caused by multipath in ZTD estimates. We apply several process noise models based on an Extended Kalman Filter (EKF) and study whether those models are capable of partly mitigating the multipath error. Simulated data, generated with a commercial GNSS simulator, is thereby used to study the impact of multipath signals. All results are based on PPP solutions for which ambiguities are resolved (PPP-AR) and since the simulations provide us reference data, the degradation of ZTD accuracy can be studied in different scenarios where the multipath errors come from different reflection sources. The results reveal that the magnitude of ZTD errors due to multipath reaches millimeter to centimeter order, depending on the chosen scenario. In order to mitigate the effect of multipath errors on ZTD estimates, we study the use of the Continuous Wavelet Transform (CWT). We compute Code-minus-Carrier (CMC) observations and apply a CWT with the purpose to detect the periods during which multipath errors are affecting the observables. Once those periods are identified, it might be possible to mitigate the multipath error by using different process noise models or different function representations for the unknown parameters. In particular, we focus on the random walk model, the first-order Gaussian-Markov model, a noise-overbounding approach and a B-spline representation. We discuss how effective those models perform and reveal whether there is the possibility to improve troposphere estimates which would otherwise be biased by multipath effects. &#160;Although all our findings relate to PPP post-processing, the suggested approaches can be mapped to real-time applications since multipath errors mitigation is done epoch wise with the help of an EKF.</p>
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