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A Two-Component Regulator of Universal Stress Protein Expression and Adaptation to Oxygen Starvation in Mycobacterium smegmatis. Ronan O'Toole, 2003.We identified a response regulator in Mycobacterium smegmatis which plays an important role in adaptation to oxygen-starved stationary phase . The regulator exhibits strong sequence similarity to DevR/Rv3133c of M . tuberculosis . The structural gene is present on a multigene locus, which also encodes a sensor kinase . A devR mutant of M . smegmatis was adept at surviving growth arrest initiated by either carbon or nitrogen starvation . However, its culturability decreased several orders of magnitude below that of the wild type under oxygen-starved stationary-phase conditions . Two-dimensional gel analysis revealed that a number of oxygen starvation-inducible proteins were not expressed in the devR mutant . Three of these proteins are universal stress proteins, one of which is encoded directly upstream of devR . Another protein closely resembles a proposed nitroreductase, while a fifth protein corresponds to the  -crystallin (HspX) orthologue of M . smegmatis . None of the three universal stress proteins or nitroreductase, and a considerably lower amount of HspX was detected in carbon-starved wild-type cultures . A fusion of the hspX promoter to gfp demonstrated that DevR directs gene expression when M . smegmatis enters stationary phase brought about, in particular, by oxygen starvation . To our knowledge, this is the first time a role for a two-component response regulator in the control of universal stress protein expression has been shown . Notably, the devR mutant was 104-fold more sensitive than wild type to heat stress . We conclude that DevR is a stationary-phase regulator required for adaptation to oxygen starvation and resistance to heat stress in M . smegmatis .
Hydrogen-Dependent Oxygen Reduction by Homoacetogenic Bacteria Isolated from Termite Guts. Hamadi I. Boga, 2003.Although homoacetogenic bacteria are generally considered to be obligate anaerobes, they colonize the intestinal tracts of termites and other environments that are not entirely anoxic in space or time . In this study, we investigated how homoacetogenic bacteria isolated from the hindguts of various termites respond to the presence of molecular oxygen . All strains investigated formed growth bands in oxygen gradient agar tubes under a headspace of H2-CO2 . The position of the bands coincided with the oxic-anoxic interface and depended on the O2 partial pressure in the headspace; the position of the bands relative to the meniscus remained stable for more than 1 month . Experiments with dense cell suspensions, performed with Clark-type O2 and H2 electrodes, revealed a large capacity for H2-dependent oxygen reduction in Sporomusa termitida and Sporomusa sp . strain TmAO3 (149 and 826 nmol min-1 mg of protein-1, respectively) . Both strains also reduced O2 with endogenous reductants, albeit at lower rates . Only in Acetonema longum did the basal rates exceed the H2-dependent rates considerably (181 versus 28 nmol min-1 mg of protein)-1) . Addition of organic substrates did not stimulate O2 consumption in any of the strains . Nevertheless, reductive acetogenesis by cell suspensions of strain TmAO3 was inhibited even at the lowest O2 fluxes, and growth in nonreduced medium occurred only after the bacteria had rendered the medium anoxic . Similar results were obtained with Acetobacterium woodii, suggesting that the results are not unique to the strains isolated from termites . We concluded that because of their tolerance to temporary exposure to O2 at low partial pressures (up to 1.5 kPa in the case of strain TmAO3) and because of their large capacity for O2 reduction, homoacetogens can reestablish conditions favorable for growth by actively removing oxygen from their environment .
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