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Consequences of a Deletion in dspA on Transcript Accumulation in Synechocystis sp . Strain PCC6803. Chao-Jung Tu, 2004.A sensor histidine kinase of Synechococcus sp . strain PCC7942, designated nblS, was previously identified and shown to be critical for the acclimation of cells to high-light and nutrient limitation conditions and to influence the expression of a number of light-responsive genes . The nblS orthologue in Synechocystis sp . strain PCC6803 is designated dspA (also called hik33) . We have generated a dspA null mutant and analyzed global gene expression in both the mutant and wild-type strains under high- and low-light conditions . The mutant is aberrant for the expression of many genes encoding proteins critical for photosynthesis, phosphate and carbon acquisition, and the amelioration of stress conditions . Furthermore, transcripts from a number of genes normally detected only during exposure of wild-type cells to high-light conditions become partially constitutive in the low-light-grown dspA mutant . Other genes for which transcripts decline upon exposure of wild-type cells to high light are already lower in the mutant during growth in low light . These results suggest that DspA may influence gene expression in both a positive and a negative manner and that the dspA mutant behaves as if it were experiencing stress conditions (e.g., high-light exposure) even when maintained at near-optimal growth conditions for wild-type cells . This is discussed with respect to the importance of DspA for regulating the responses of the cell to environmental cues . Cooperativity in Signal Transfer through the Uhp System of Escherichia coli. Daniël T. Verhamme, 2002.The UhpABC regulatory system in enterobacteria controls the expression of the hexose phosphate transporter UhpT . Signaling is initiated through sensing of extracellular glucose 6-phosphate by membrane-bound UhpC, which in turn modulates the histidine-protein kinase UhpB . Together with the cytoplasmic response regulator UhpA, they constitute a typical two-component regulatory system based on His-to-Asp phosphoryl transfer . Activated (i.e., phosphorylated) UhpA binds to the promoter region of uhpT, resulting in initiation of transcription . We have investigated the contribution of transmembrane signaling (through UhpBC) and intracellular activation (through UhpA) to the overall Uhp response (UhpT expression) in vivo . UhpA activation could be made independent of transmembrane signaling when  uhpBC cells were grown on pyruvate . Inorganic phosphate interfered with glucose 6-phosphate-dependent, UhpBC-mediated, as well as pyruvate-mediated activation of UhpA . The relationship between the concentration of inducer (glucose 6-phosphate) and the Uhp induction rate was nonhyperbolic, indicating positive cooperativity . The degree of cooperativity was affected by the carbon or energy source available to the cells for growth . As pyruvate-mediated activation of UhpA in
uhpBC cells could result in considerably stronger UhpT expression than glucose 6-phosphate-dependent activation through UhpBC, the observed positive cooperativity for the overall pathway in wild-type cells may reflect the previously described cooperative binding of UhpA to the uhpT promoter (J . L . Dahl et al., J . Biol . Chem . 272:1910-1919, 1997) .
Comparison of Velocity Profiles for Different Flow Chamber Designs Used in Studies of Microbial Adhesion to Surfaces. D. P. Bakker, 2003.Flow chambers are commonly used to study microbial adhesion to surfaces under environmentally relevant hydrodynamic conditions . The parallel plate flow chamber (PPFC) is the most common design, and mass transport occurs through slow convective diffusion . In this study, we analyzed four different PPFCs to determine whether the expected hydrodynamic conditions, which control both mass transport and detachment forces, are actually achieved . Furthermore, the different PPFCs were critically evaluated based on the size of the area where the velocity profile was established and constant with a range of flow rates, indicating that valid observations could be made . Velocity profiles in the different chambers were calculated by using a numerical simulation model based on the finite element method and were found to coincide with the profiles measured by particle image velocimetry . Environmentally relevant shear rates between 0 and 10,000 s–1 could be measured over a sizeable proportion of the substratum surface for only two of the four PPFCs . Two models appeared to be flawed in the design of their inlets and outlets and allowed development of a stable velocity profile only for shear rates up to 0.5 and 500 s–1 . For these PPFCs the inlet and outlet were curved, and the modeled shear rates deviated from the calculated shear rates by up to 75% . We concluded that PPFCs used for studies of microbial adhesion to surfaces should be designed so that their inlets and outlets are in line with the flow channel . Alternatively, the channel length should be increased to allow a greater length for the establishment of the desired hydrodynamic conditions .
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