Hydrogenases of Desulfovibrio desulfuricans G20

The sulfate-reducing bacteria (SRB) are a diverse group of anaerobic microorganisms defined primarily by the ability to utilize inorganic sulfate as a terminal electron acceptor. SRB are linked with environmental biocorrosion of ferrous metals. The metabolism of environmental pollutants, such as harmful organic compounds, metals, and radionuclides, by SRB offers a potential route to bioremediation. Hydrogen metabolism is believed to be essential for both biocorrosion and bioremediation. Hydrogenases catalyze the reversible oxidation of hydrogen and perform physiological functions central to the metabolism of the SRB. To explore the roles of the hydrogenases in the SRB, the operons encoding [NiFe] and [Fe] hydrogenases from the SRB Desulfovibrio desulfuricans G20 were cloned for this research. Portions of the cloned genes were then used to construct specific hydrogenase mutants. The growth, biocorrosion rates, and heavy metal resistance of well-defined hydrogenase mutants of D. desulfuricans G20 and the regulation of the hydrogenase operons were investigated.; A better understanding of the physiological roles and regulation of the hydrogenases was obtained from this research. The level of heavy metal tolerance by the hydrogenase mutants and the wild-type D. desulfuricans G20 were similar for CrO₄−2, SeO₄ −2, AL(III), Cu(II), and Pb(II). However, the mutants had a slightly higher resistance to Ni(II) and MoO₄−2 than wild type. This information should be useful for bioremediation studies to determine if the organism would be expected to survive in heavy metal contaminated environments and be useful for genetic engineering to better decontaminate these environments.; Little effect of the mutations was observed on growth rates on several commonly used substrates, including hydrogen. Total cellular hydrogenase activity was essentially unimpaired by the mutation of the [NiFe] hydrogenase isozyme. The periplasmic hydrogenase activity in the [Fs] hydrogenase mutant was an eighth of that of the wildtype cells. Surprisingly, one [NiFe] hydrogenase mutant (JAR6) had five times the periplasmic activity of wild-type cells. Apparently, theses enzymes were not essential under the laboratory growth conditions selected for investigation. Expression of the [Fe] hydrogenase operon (hydAB) decreased with cell growth, and was independent of the presence or absence of hydrogen.; In summary, clear physiological roles for [Fe] or [NiFe] hydrogenase in Desulfovibrio desulfuricans G20 have yet to be established.