Amphibacillus xylanus EpOl, a facultative anaerobe we recently isolated, shows rapid aerobic growth even though it lacks a respiratory pathway. Thus, the oxidative consumption of NADH, produced during glycolysis and pyruvate oxidation, should be especially important for maintenance of intracellular redox balance in this bacterium. We purified a flavoprotein functional as NADH oxidase from aerobically growing A. xylanus EpOl.The A. xylanus enzyme is a homotetramer composed of a subunit (Mr 56,000) containing 1 mol of fiavin adenine dinucleotide. This enzyme catalyzes the reduction of oxygen to hydrogen peroxide with ,-NADH as the preferred electron donor and exhibits no activity with NADPH. The Hlavoprotein gene of A. xylanus EpOl was cloned by using a specific antibody. The amino acid sequence of 509 residues, deduced from the nucleotide sequence, showed 51.2 and 72.5% identities to the amino acid sequences of alkyl hydroperoxide reductase from Salmonella typhimurium and NADH dehydrogenase from alkalophilic Bacillus sp. strain YN-1, respectively.Bacillus spp. have a respiratory chain and grow well under aerobic conditions. In contrast, Amphibacillus spp., having no respiratory chain, grow equally well under both aerobic and anaerobic conditions, which distinguishes these two genera. Salmonella spp., which are gram-negative bacteria, are taxonomically distant from gram-positive bacteria such as Bacillus spp. and Amphibacillus spp. The above findings, however, suggest that the flavoprotein functional as NADH oxidase, the alkyl hydroperoxide reductase, and the NADH dehydrogenase diverged recently, with only small changes leading to their functional differences.We recently isolated a new group of facultatively anaerobic bacteria from an alkaline compost which have unique phenotypic and chemotaxonomic characteristics (17), as well as bioenergetic properties (10). The isolates were then recognized to belong to an new taxon and named Amphibacillus xylanus (16).In aerobic and facultatively anaerobic bacterial cells having a respiratory pathway, most of the NADH produced through glycolysis and the tricarboxylic acid cycle is effectively utilized to generate biological energy by the respiratory chain. A. xylanus, lacking a respiratory system, grows well and has the same growth rate and cell yield under strictly anaerobic conditions and aerobic conditions with shaking (16). This growth characteristic of A. xylanus appears to be due to the presence of pathways that function under both anaerobic and aerobic conditions and produce similar amounts of ATP; NADH produced anaerobically through glycolysis is reoxidized by NAD-linked aldehyde dehydrogenase and NAD-linked alcohol dehydrogenase (15). Under aerobic conditions, the high NADH oxidase activity which is detected in a cell extract should be responsible for maintenance of the intracellular redox balance and thus assuring fast aerobic growth by consuming NADH (15).
* Corresponding author.NADH oxidase is found in several microorganisms (7,12,18,23) and has been purified from ...
