This study shows that the Bacillus anthracis pXO1 virulence plasmid carries a Rap-Phr system, BXA0205, which regulates sporulation initiation in this organism. The BXA0205Rap protein was shown to dephosphorylate the Spo0F response regulator intermediate of the phosphorelay signal transduction system that regulates the initiation of the developmental pathway in response to environmental, metabolic, and cell cycle signals. The activity of the Rap protein was shown to be inhibited by the carboxy-terminal pentapeptide generated through an export-import processing pathway from the associated BXA0205Phr protein. Deregulation of the Rap activity by either overexpression or lack of the Phr pentapeptide resulted in severe inhibition of sporulation. Five additional Rap-Phr encoding systems were identified on the chromosome of B. anthracis, one of which, BA3790-3791, also affected sporulation initiation. The results suggest that the plasmid-borne Rap-Phr system may provide a selective advantage to the virulence of B. anthracis.Bacillus anthracis, the etiological agent of anthrax, is a grampositive spore-forming organism that primarily infects ruminants but can also be highly pathogenic to other mammals, including humans. The intrinsic spore resistance to extreme stresses such as desiccation, solvents, extreme pH, temperature, UV, and ionizing radiation plays a major role in anthrax pathogenesis. Sporulation is essential for survival in the environment, and it evidently contributes to anthrax diffusion, because spores are usually present when the infection is initiated (27).The process of sporulation has been extensively studied in Bacillus subtilis and shown to be the result of a complex differentiation pathway that has its onset in a signal transduction system called phosphorelay. The phosphorelay is a more complex version of the two-component signal transduction systems, because it is composed of multiple central elements and it is modulated by a variety of ancillary factors (40,41).In B. subtilis, five histidine sensor kinases (KinA, -B, -C, -D, and -E) can respond to a multiplicity of inducing signals and activate the pathway by autophosphorylating and transferring the activating phosphoryl group to an intermediate response regulator acceptor called Spo0F. From Spo0F, the phosphoryl group is then transferred to the Spo0A response regulator through the Spo0B phosphotransferase. Spo0A is the critical transcription regulator for sporulation initiation. Accumulation of its activated form, Spo0AϳP, during growth progressively results in the repression of genes not required for sporulation and the activation of genes necessary for spore formation (7,19,28,53).Negative inputs into the phosphorelay are brought about mainly by aspartyl phosphate phosphatases that specifically dephosphorylate the response regulator components of the system. The three members of the Spo0E family of phosphatases dephosphorylate Spo0AϳP, while three members of the Rap family of phosphatases dephosphorylate the Spo0FϳP intermediate (18,36,38). Rap p...
Marine coccolithophorid algae are thought to play a significant role in carbon cycling due to their ability to incorporate dissolved inorganic carbon (DIC) into both calcite and photosynthetic products. Among coccolithophorids, Emiliania huxleyi is the most prolific, forming massive blooms that affect the global environment. In addition to its ecological importance, the elaborate calcite structures (coccoliths) are being investigated for the design of potential materials for science and biotechnological devices. To date, most of the research focus in this organism has involved the partitioning of DIC between calcification and photosynthesis, primarily using measurements of an external versus internal carbonic anhydrase (CA) activity under defined conditions. The actual genes, proteins, and pathways employed in these processes have not been identified and characterized Single-celled marine algae fix inorganic carbon via the Calvin-Benson-Bassham cycle, resulting in the formation of 2 mol of phosphoglyceric acid from CO 2 and its five-carbon acceptor. The enzyme responsible for this reaction, ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), requires CO 2 for carbon fixation, and kinetic analyses of these enzymes from several photosynthetic microorganisms have shown that RubisCO enzymes have poor binding affinity for this substrate. The predominant form of dissolved inorganic carbon (DIC) in the oceans is bicarbonate (ϳ2 mM), and concentrations of aqueous CO 2 are very low (10 M), well below the half-saturation constants of most marine algal RubisCO enzymes (6). This requires that unicellular algae increase intracellular DIC levels either via the direct transport of HCO 3 Ϫ or by the activity of an external carbonic anhydrase (CA) (CA ext ). All carbon-concentrating mechanisms (CCMs) described to date in marine phytoplankton involve the zinc metalloenzyme carbonic anhydrase; therefore, CO 2 acquisition via these enzymes is dependent upon the availability of trace metals (Zn, Co, and Cd), the concentrations of which are extremely low (nanomolar to picomolar levels) in ocean surface waters. These conditions place severe constraints on CO 2 availability for marine phytoplankton via CA, yet growth kinetics and global distribution data on coccolithophorids and diatoms suggest that they are not generally CO 2 limited for photosynthesis under most environmental conditions. Considering the low concentrations and dramatic fluctuations in these trace metals, it is logical to envision scenarios under which CA activity is repressed and an alternative CCM mechanism is induced, such as the CCM pathways observed in C 4 and Crassulacean acid metabolism (CAM) plants. An example of such a mechanism was recently described for the marine diatom Thalassiosira weissflogii. The results of that elegant study provided solid evidence for C 4 photosynthesis in a unicellular alga (26). Another study showed a similar C 4 mechanism within a single photosynthetic cell in a terrestrial plant (44), and thus, the argument requiring a Kran...
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