Chemolithoautotrophic assimilation of dinitrogen by Streptomyces thermoautotrophicus UBT1: identification of an unusual N2-fixing system

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Abstract
Streptomyces thermoautotrophicus UBT1, which was isolated previously from a burning charcoal pile, was shown to utilize N2 as a sole nitrogen source when growing chemolithoautotrophically with CO or H2 plus CO2 under aerobic conditions at 65 degrees C. Doubling times under diazotrophic conditions were 10 h. S. thermoautotrophicus is a new CO- or H2-oxidizing, obligately chemolithoautotrophic, thermophilic, free-living, aerobic, N2-fixing streptomycete. Its ability to fix N2 was also evident from (i) the incorporation of substantial amounts of 15N2 (about 13%) into cell material, (ii) the formation of H2 during diazotrophic growth, (iii) the repression of 15N2 assimilation and H2 formation by ammonia, and (iv) culture growth yields with N2 as a nitrogen source that were significantly higher than those without any added nitrogen compounds (ca. 2.4 versus < 0.1 mg [dry weight]). The N2-fixing system of S. thermoautotrophicus exhibited several properties not apparent in the diazotrophic bacteria studied so far, since it was (i) incapable of reducing acetylene to ethylene or ethane and (ii) resistant to inhibition by acetylene or ethylene (5% [vol/vol] each), CO (40 to 70% [vol/vol]), or H2 (40% [vol/vol]). Under stringent conditions, nifH and nifDK gene probes from Klebsiella pneumoniae did not hybridize with total DNA from S. thermoautotrophicus.