Only five bacterial phyla with members capable of chlorophyll (Chl)-based phototrophy are presently known. Metagenomic data from the phototrophic microbial mats of alkaline siliceous hot springs in Yellowstone National Park revealed the existence of a distinctive bacteriochlorophyll (BChl)-synthesizing, phototrophic bacterium. A highly enriched culture of this bacterium grew photoheterotrophically, synthesized BChls a and c under oxic conditions, and had chlorosomes and type 1 reaction centers. "Candidatus Chloracidobacterium thermophilum" is a BChl-producing member of the poorly characterized phylum Acidobacteria.

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The ptxD gene from Pseudomonas stutzeri WM88 encoding the novel phosphorus oxidizing enzyme NAD:phosphite oxidoreductase (trivial name phosphite dehydrogenase, PtxD) was cloned into an expression vector and overproduced in Escherichia coli. The heterologously produced enzyme is indistinguishable from the native enzyme based on mass spectrometry, amino-terminal sequencing, and specific activity analyses. Recombinant PtxD was purified to homogeneity via a two-step affinity protocol and characterized. The enzyme stoichiometrically produces NADH and phosphate from NAD and phosphite. The reverse reaction was not observed. Gel filtration analysis of the purified protein is consistent with PtxD acting as a homodimer. PtxD has a high affinity for its substrates with K m values of 53.1 ؎ 6.7 M and 54.6 ؎ 6.7 M, for phosphite and NAD, respectively. V max and k cat were determined to be 12.2 ؎ 0.3 mol min ؊1 mg ؊1 and 440 min ؊1. NADP can substitute poorly for NAD; however, none of the numerous compounds examined were able to substitute for phosphite. Initial rate studies in the absence or presence of products and in the presence of the dead end inhibitor sulfite are most consistent with a sequential ordered mechanism for the PtxD reaction, with NAD binding first and NADH being released last. Amino acid sequence comparisons place PtxD as a new member of the D-2-hydroxyacid NAD-dependent dehydrogenases, the only one to have an inorganic substrate. To our knowledge, this is the first detailed biochemical study on an enzyme capable of direct oxidation of a reduced phosphorus compound.Phosphorus is widely reported to be a redox conservative element in biological systems, with the sum total of phosphorus biochemistry consisting of the formation and hydrolysis of phosphate-ester bonds. These reports imply that reduced phosphorus compounds are not important in living systems and that enzymatically catalyzed redox reactions of phosphorus compounds do not occur; however, an increasing body of evidence indicates that this is not the case. Although it is true that inorganic phosphate (P valence ϩ5) is the principal form of phosphorus in living systems and that phosphate-esters play a critical role in phosphate biochemistry, it is now clear that reduced phosphorus compounds of both natural and xenobiotic origin play important roles in numerous biological systems. Accordingly, many organisms have been shown to possess metabolic pathways for reduction of phosphate to a variety of reduced phosphorus compounds (1-3); others have been shown to possess metabolic pathways for oxidation of reduced phosphorus compounds (4 -9). Among the most striking of these is a recently isolated sulfate-reducing bacterium that obtains all of the energy it requires for growth from the oxidation of phosphite (ϩ3 valence) to phosphate (10).Unfortunately, detailed studies examining the mechanisms of biological phosphorus oxidation and reduction are scarce. This is particularly true with regard to the biochemical characterization of enzymes involved in reduced p...