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Effects of Perturbing Nucleoid Structure on Nucleoid Occlusion-Mediated Toporegulation of FtsZ Ring Assembly. Qin Sun, 2004.In Escherichia coli, assembly of the FtsZ ring (Z ring) at the cell division site is negatively regulated by the nucleoid in a phenomenon called nucleoid occlusion (NO) . Previous studies have indicated that chromosome packing plays a role in NO, as mukB mutants grown in rich medium often exhibit FtsZ rings on top of diffuse, unsegregated nucleoids . To address the potential role of overall nucleoid structure on NO, we investigated the effects of disrupting chromosome structure on Z-ring positioning . We found that NO was mostly normal in cells with inactivated DNA gyrase or in mukB-null mutants lacking topA, although some suppression of NO was evident in the latter case . Previous reports suggesting that transcription, translation, and membrane insertion of proteins ("transertion") influence nucleoid structure prompted us to investigate whether disruption of these activities had effects on NO . Blocking transcription caused nucleoids to become diffuse, and FtsZ relocalized to multiple bands on top of these nucleoids, biased towards midcell . This suggested that these diffuse nucleoids were defective in NO . Blocking translation with chloramphenicol caused characteristic nucleoid compaction, but FtsZ rarely assembled on top of these centrally positioned nucleoids . This suggested that NO remained active upon translation inhibition . Blocking protein secretion by thermoinduction of a secA(Ts) strain caused a chromosome segregation defect similar to that in parC mutants, and NO was active . Although indirect effects are certainly possible with these experiments, the above data suggest that optimum NO activity may require specific organization and structure of the nucleoid . Influence of Humic Substances on Bacterial and Viral Dynamics in Freshwaters. Alexandre M. Anesio, 2004.Bacterial and viral abundances were measured in 24 lakes with dissolved organic carbon (DOC) concentrations ranging from 3 to 19 mg of C liter–1 . In addition, a laboratory experiment was performed to test the effects of different sources of carbon (i.e., glucose and fulvic acids) and nutrients on the dynamics of viruses and bacteria . In the lake survey, no correlation was found between virus abundance and DOC concentration, yet there was a significant positive correlation between bacterial abundance and DOC concentration . A negative correlation was found between the virus-to-bacteria ratio and DOC level . These results are in agreement with our findings in the laboratory, where virus counts were significantly lower in treatments with fulvic acid additions than in a control (mean, 67.4% ± 6.5% of the control) . Virus counts did not differ significantly among the control and treatments with glucose, indicating that it was the type of organic carbon and not quantity which had an impact on viruses . Results from this study suggest that the way viruses control bacterial assemblages in humic lakes is different from the mechanism in clear water systems . Molecular Characterization and Transcriptional Analysis of adhE2, the Gene Encoding the NADH-Dependent Aldehyde/Alcohol Dehydrogenase Responsible for Butanol Production in Alcohologenic Cultures of Clostridium acetobutylicum ATCC 824. Lisa Fontaine, 2002.The adhE2 gene of Clostridium acetobutylicum ATCC 824, coding for an aldehyde/alcohol dehydrogenase (AADH), was characterized from molecular and biochemical points of view . The 2,577-bp adhE2 codes for a 94.4-kDa protein . adhE2 is expressed, as a monocistronic operon, in alcohologenic cultures and not in solventogenic cultures . Primer extension analysis identified two transcriptional start sites 160 and 215 bp upstream of the adhE2 start codon . The expression of adhE2 from a plasmid in the DG1 mutant of C . acetobutylicum, a mutant cured of the pSOL1 megaplasmid, restored butanol production and provided elevated activities of NADH-dependent butyraldehyde and butanol dehydrogenases . The recombinant AdhE2 protein expressed in E . coli as a Strep-tag fusion protein and purified to homogeneity also demonstrated NADH-dependent butyraldehyde and butanol dehydrogenase activities . This is the second AADH identified in C . acetobutylicum ATCC 824, and to our knowledge this is the first example of a bacterium with two AADHs . It is noteworthy that the two corresponding genes, adhE and adhE2, are carried by the pSOL1 megaplasmid of C . acetobutylicum ATCC 824 . Hydrogenotrophic Methanogenesis by Moderately Acid-Tolerant Methanogens of a Methane-Emitting Acidic Peat. Marcus A. Horn, 2003.The emission of methane (1.3 mmol of CH4 m-2 day-1), precursors of methanogenesis, and the methanogenic microorganisms of acidic bog peat (pH 4.4) from a moderately reduced forest site were investigated by in situ measurements, microcosm incubations, and cultivation methods, respectively . Bog peat produced CH4 (0.4 to 1.7 µmol g [dry wt] of soil-1 day-1) under anoxic conditions . At in situ pH, supplemental H2-CO2, ethanol, and 1-propanol all increased CH4 production rates while formate, acetate, propionate, and butyrate inhibited the production of CH4; methanol had no effect . H2-dependent acetogenesis occurred in H2-CO2-supplemented bog peat only after extended incubation periods . Nonsupplemented bog peat initially produced small amounts of H2 that were subsequently consumed . The accumulation of H2 was stimulated by ethanol and 1-propanol or by inhibiting methanogenesis with bromoethanesulfonate, and the consumption of ethanol was inhibited by large amounts of H2; these results collectively indicated that ethanol- or 1-propanol-utilizing bacteria were trophically associated with H2-utilizing methanogens . A total of 109 anaerobes and 107 hydrogenotrophic methanogens per g (dry weight) of bog peat were enumerated by cultivation techniques . A stable methanogenic enrichment was obtained with an acidic, H2-CO2-supplemented, fatty acid-enriched defined medium . CH4 production rates by the enrichment were similar at pH 4.5 and 6.5, and acetate inhibited methanogenesis at pH 4.5 but not at pH 6.5 . A total of 27 different archaeal 16S rRNA gene sequences indicative of Methanobacteriaceae, Methanomicrobiales, and Methanosarcinaceae were retrieved from the highest CH4-positive serial dilutions of bog peat and methanogenic enrichments . A total of 10 bacterial 16S rRNA gene sequences were also retrieved from the same dilutions and enrichments and were indicative of bacteria that might be responsible for the production of H2 that could be used by hydrogenotrophic methanogens . These results indicated that in this acidic bog peat, (i) H2 is an important substrate for acid-tolerant methanogens, (ii) interspecies hydrogen transfer is involved in the degradation of organic carbon, (iii) the accumulation of protonated volatile fatty acids inhibits methanogenesis, and (iv) methanogenesis might be due to the activities of methanogens that are phylogenetic members of the Methanobacteriaceae, Methanomicrobiales, and Methanosarcinaceae .
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