Utilization of carbon monoxide by aerobes: recent advances

Abstract

Abstract The list of carboxydotrophic bacteria is constantly growing and we have found that burning charcoal piles harbor an especially rich CO-oxidizing microflora. The newly isolated Streptomyces thermoautotrophicus UBT1 is particularly interesting as it is thermophilic, capable of chemolithoautotrophic growth with CO or H 2 plus CO 2 and incapable of using organic substrates. Molybdenum is essential for CO-autotrophic growth. Some species of carboxydotrophic bacteria can denitrify under heterotrophic conditions yielding N 2 (e.g. Pseudomonas carboxydoflava ) or N 2 O (e.g. Pseudomonas carboxydohydrogena ); others perform nitrate respiration (e.g. Azomonas B1). P. carboxydohydrogena could grow at the expense of H 2 plus CO 2 using nitrate as electron acceptor. In intact cells of Pseudomonas carboxydovorans , CO dehydrogenase has the ability of dissociating from and rebinding to the cytoplasmic membrane. That process can be simulated in vitro by removing CO dehydrogenase from cytoplasmic membranes and rebinding it to depleted membranes. Reconstitution of the enzyme onto depleted membranes requiring di- or trivalent cations, was specific for membranes from CO-grown bacteria and led to reactivation of respiratory activities with CO. A complex consisting of 1 molecule of CO dehydrogenase and 2 molecules of cytochrome b 561 could be isolated from cytoplasmic membranes of P. carboxydovorans solubilized with dodecyl β- d -maltoside. Within the complex as well as in assays containing purified CO dehydrogenase and cytochrome b 561 the latter could serve as an electron acceptor. CO dehydrogenase had hydrogenase activity, and its K M of only 5 mM H 2 suggested a role in the formation of H 2 . P. carboxydovorans OM5 contains the 128-kilo-base pairs (kb) plasmid pHCG3 which is essential for CO- and H 2 -lithoautotrophic growth. Evidence for the existence of pHCG3-coded structural genes of CO dehydrogenase was obtained from dot blot hybridizations employing synthetic oligodeoxynucleotides as heterologous probes for the detection of the S- and M-subunit genes. Employing appropriate probe genes encoding membrane-bound hydrogenase, ribulose biphosphate carboxylase and phosphoribulokinase were also identified on plasmid pHCG3.