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Codenitrification and Denitrification Are Dual Metabolic Pathways through Which Dinitrogen Evolves from Nitrate in Streptomyces antibioticus. Yasuyuki Kumon, 2002.We screened actinomycete strains for dinitrogen (N2)-producing activity and discovered that Streptomyces antibioticus B-546 evolves N2 and some nitrous oxide (N2O) from nitrate (NO3-) . Most of the N2 that evolved from the heavy isotope ([15N]NO3-) was 15N14N, indicating that this nitrogen species consists of two atoms, one arising from NO3- and the other from different sources . This phenomenon is similar to codenitrification in fungi . The strain also evolved less, but significant, amounts of 15N15N from [15N]NO3- in addition to 15N15NO with concomitant cell growth . Prior to the production of N2 and N2O, NO3- was rapidly reduced to nitrite (NO2-) accompanied by distinct cell growth, showing that the actinomycete strain is a facultative anaerobe that depends on denitrification and nitrate respiration for anoxic growth . The cell-free activities of denitrifying enzymes could be reconstituted, supporting the notion that the 15N15N and 15N15NO species are produced by denitrification from NO3- via NO2- . We therefore demonstrated a unique system in an actinomycete that produces gaseous nitrogen (N2 and N2O) through both denitrification and codenitrification . The predominance of codenitrification over denitrification along with oxygen tolerance is the key feature of nitrate metabolism in this actinomycete . ATP-Bound Conformation of Topoisomerase IV: a Possible Target for Quinolones in Streptococcus pneumoniae. Farid Sifaoui, 2003.Topoisomerase IV, a C2E2 tetramer, is involved in the topological changes of DNA during replication . This enzyme is the target of antibacterial compounds, such as the coumarins, which target the ATP binding site in the ParE subunit, and the quinolones, which bind, outside the active site, to the quinolone resistance-determining region (QRDR) . After site-directed and random mutagenesis, we found some mutations in the ATP binding site of ParE near the dimeric interface and outside the QRDR that conferred quinolone resistance to Streptococcus pneumoniae, a bacterial pathogen . Modeling of the N-terminal, 43-kDa ParE domain of S . pneumoniae revealed that the most frequent mutations affected conserved residues, among them His43 and His103, which are involved in the hydrogen bond network supporting ATP hydrolysis, and Met31, at the dimeric interface . All mutants showed a particular phenotype of resistance to fluoroquinolones and an increase in susceptibility to novobiocin . All mutations in ParE resulted in resistance only when associated with a mutation in the QRDR of the GyrA subunit . Our models of the closed and open conformations of the active site indicate that quinolones preferentially target topoisomerase IV of S . pneumoniae in its ATP-bound closed conformation .
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