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Analysis of Chimeric Chemoreceptors in Bacillus subtilis Reveals a Role for CheD in the Function of the McpC HAMP Domain. Christopher J. Kristich, 2004.Motile prokaryotes use a sensory circuit for control of the motility apparatus in which ligand-responsive chemoreceptors regulate phosphoryl flux through a modified two-component signal transduction system . The chemoreceptors exhibit a modular architecture, comprising an N-terminal sensory module, a C-terminal output module, and a HAMP domain that connects the N- and C-terminal modules and transmits sensory information between them via an unknown mechanism . The sensory circuits mediated by two chemoreceptors of Bacillus subtilis have been studied in detail . McpB is known to regulate chemotaxis towards the attractant asparagine in a CheD-independent manner, whereas McpC requires CheD to regulate chemotaxis towards the attractant proline . Although CheD is a phylogenetically widespread chemotaxis protein, there exists only a limited understanding of its function . We have constructed chimeras between McpB and McpC to probe the role of CheD in facilitating sensory transduction by McpC . We found that McpC can be converted to a CheD-independent receptor by the replacement of one-half of its HAMP domain with the corresponding sequence from McpB, suggesting that McpC HAMP domain function is complex and may require intermolecular interactions with the CheD protein . When considered in combination with the previous observation that CheD catalyzes covalent modification of the C-terminal modules of B . subtilis receptors, these results suggest that CheD may interact with chemoreceptors at multiple, functionally distinct sites . Effect of Nematodes on Rhizosphere Colonization by Seed-Applied Bacteria. Oliver G. G. Knox, 2004.There is much interest in the use of seed-applied bacteria for biocontrol and biofertilization, and several commercial products are available . However, many attempts to use this strategy fail because the seed-applied bacteria do not colonize the rhizosphere . Mechanisms of rhizosphere colonization may involve active bacterial movement or passive transport by percolating water or plant roots . Transport by other soil biota is likely to occur, but this area has not been well studied . We hypothesized that interactions with soil nematodes may enhance colonization . To test this hypothesis, a series of microcosm experiments was carried out using two contrasting soils maintained under well-defined physical conditions where transport by mass water flow could not occur . Seed-applied Pseudomonas fluorescens SBW25 was capable of rhizosphere colonization at matric potentials of –10 and –40 kPa in soil without nematodes, but colonization levels were substantially increased by the presence of nematodes . Our results suggest that nematodes can have an important role in rhizosphere colonization by bacteria in soil . Novel Organization and Divergent Dockerin Specificities in the Cellulosome System of Ruminococcus flavefaciens. Marco T. Rincon, 2003.The DNA sequence coding for putative cellulosomal scaffolding protein ScaA from the rumen cellulolytic anaerobe Ruminococcus flavefaciens 17 was completed . The mature protein exhibits a calculated molecular mass of 90,198 Da and comprises three cohesin domains, a C-terminal dockerin, and a unique N-terminal X domain of unknown function . A novel feature of ScaA is the absence of an identifiable cellulose-binding module . Nevertheless, native ScaA was detected among proteins that attach to cellulose and appeared as a glycosylated band migrating at around 130 kDa . The ScaA dockerin was previously shown to interact with the cohesin-containing putative surface-anchoring protein ScaB . Here, six of the seven cohesins from ScaB were overexpressed as histidine-tagged products in E . coli; despite their considerable sequence differences, each ScaB cohesin specifically recognized the native 130-kDa ScaA protein . The binding specificities of dockerins found in R . flavefaciens plant cell wall-degrading enzymes were examined next . The dockerin sequences of the enzymes EndA, EndB, XynB, and XynD are all closely related but differ from those of XynE and CesA . A recombinant ScaA cohesin bound selectively to dockerin-containing fragments of EndB, but not to those of XynE or CesA . Furthermore, dockerin-containing EndB and XynB, but not XynE or CesA, constructs bound specifically to native ScaA . XynE- and CesA-derived probes did however bind a number of alternative R . flavefaciens bands, including an  110-kDa supernatant protein expressed selectively in cultures grown on xylan . Our findings indicate that in addition to the ScaA dockerin-ScaB cohesin interaction, at least two distinct dockerin-binding specificities are involved in the novel organization of plant cell wall-degrading enzymes in this species and suggest that different scaffoldins and perhaps multiple enzyme complexes may exist in R . flavefaciens .
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