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An exopolyphosphatase [exopoly(P)ase; EC 3.6.1.11] activity has recently been purified to homogeneity from a mutant strain of Escherichia coli which lacks the principal exopoly(P)ase. The second exopoly(P)ase has now been identified as guanosine pentaphosphate phosphohydrolase (GPP; EC 3.6.1.40) by three lines of evidence: (i) the sequences of five tryptic digestion fragments of the purified protein are found in the translated gppA gene, (ii) the size of the protein (100 kDa) agrees with published values for GPP, and (iii) the ratio of exopoly(P)ase activity to GPP activity remains constant throughout a 300-fold purification in the last steps of the procedure. The enzyme liberates orthophosphate by processive hydrolysis of the phosphoanyhydride bonds of polyphosphate [poly(P)] chains (1000 residues) or by hydrolysis of the 5'-gamma-phosphate of guanosine 5'-triphosphate 3'-diphosphate (pppGpp) to guanosine 5'-diphosphate 3'-diphosphate (ppGpp or "magic spot"). The Km for long-chain poly(P) as a substrate (approximately 0.5 nM) is far lower than that for pppGpp (0.13 mM); the kcat for the poly(P)ase activity is 1.1 s-1 and that for pppGpp hydrolase is 0.023 s-1. These and other findings direct attention to possible functions of poly(P) in the response of E. coli to stresses and deprivations.
Understanding the relative contribution of environmental and substrate controls on rice paddy methanogenesis is critical for developing mechanistic models of landscape‐scale methane (CH 4 ) flux. A diurnal pattern in observed rice paddy CH 4 flux has been attributed to fluctuations in soil temperature physically driving diffusive CH 4 transport from the soil to atmosphere. Here we make direct landscape‐scale measurements of carbon dioxide and CH 4 fluxes and show that gross ecosystem photosynthesis (GEP) is the dominant cause of the diurnal pattern in CH 4 flux, even after accounting for the effects of soil temperature. The time series of GEP and CH 4 flux show strong spectral coherency throughout the rice growing season at the diurnal timescale, where the peak in GEP leads that of CH 4 flux by 1.3 ± 0.08 hours. By applying the method of conditional Granger causality in the spectral domain, we demonstrated that the diurnal pattern in CH 4 flux is primarily caused by GEP.
We have previously proposed that polysialic acid (PSA), which is attached to NCAM on the cell surface, can serve to regulate a variety of cell-cell interactions. The present study provides evidence that hydrated PSA influences a sufficiently large volume at the cell surface to exert broad steric effects, and that the removal of PSA in fact causes a detectable change in intercellular space. Using F11 neuron/neuroblastoma hybrid cells as a model system, the measured density and size of PSA suggests that a substantial fraction of the space between two apposed cell surface membranes could be sterically influenced by the presence of PSA. Specific enzymatic removal of PSA, which is similar in magnitude to changes that occur in many tissues during normal development, caused about a 25% decrease in the distance between two apposed cells. By contrast, removal of both heparan sulfate and chondroitin sulfate from the cells had no effect on this parameter. It is proposed that such changes in membrane-membrane distance could serve to alter selectively the efficiency of encounter between complementary receptors on apposing cells, and explain at least in part the broad biological influences of PSA.
Search advertising shows trends of vertical extension. Vertical ads, including product ads and local search ads, are proliferating at an ever increasing pace. They typically offer better ROI to advertisers as a result of better user engagement. However, campaigns and bids in vertical ads are not set at the keyword level. As a result, the matching between user query and ads suffers low recall rate and the match quality is heavily impacted by tail queries. In this paper, we propose an ad retrieval framework for retail vertical ads, based on query rewrite using personal history data to improve ad recall rate. To insure ad quality, we also present a relevance model for matching rewritten queries with user search intent, with a particular focus on tail queries. In addition, we designed and implemented a GPU-based system to accelerate the training of the relevance model to meet production performance constraints. Finally, we carry out extensive experiments on large-scale logs collected from Bing, and show significant gains in ad retrieval rate without compromising ad quality.
