Identification of c-type cytochromes involved in anaerobic, bacterial U(IV) oxidation

Abstract

Anaerobic, bacterial reduction of water-soluble U(VI) complexes to the poorly soluble U(IV) mineral uraninite has been intensively studied as a strategy for in situ remediation of uranium-contaminated groundwater. A novel and potentially counteracting metabolic process, anaerobic, nitrate-dependent U(IV) oxidation, has recently been described in two bacterial species (Geobacter metallireducens and Thiobacillus denitrificans), but the underlying biochemistry and genetics are completely unknown. We report here that two diheme, c-type cytochromes (putatively c(4) and c(5) cytochromes) play a major role in nitrate-dependent U(IV) oxidation by T. denitrificans. Insertion mutations in each of the two genes encoding these cytochromes resulted in a greater than 50% decrease in U(IV) oxidation activity, and complementation in trans restored activity to wild-type levels. Sucrose-density-gradient ultracentrifugation confirmed that both cytochromes are membrane-associated. Insertion mutations in genes encoding other membrane-associated, c-type cytochromes did not diminish U(IV) oxidation. This is the first report of proteins involved in anaerobic U(IV) oxidation.

The Confluence of Heavy Metal Biooxidation and Heavy Metal Resistance: Implications for Bioleaching by Extreme Thermoacidophiles

“…The latter two catalyze the nitrate-dependent oxidation of U(IV). Two di-heme, c-type cytochromes, putatively c4 and c5 cytochromes, have been found to play a major role in the nitrate-dependent U(IV) oxidation by T. denitrificans [136]. The two cytochromes are membrane-associated and may be periplasmic, based on homology to characterized c4 and c5 cytochromes in Pseudomonas stutzeri.…”

Uranium extremophily is an adaptive, rather than intrinsic, feature for extremely thermoacidophilic Metallosphaera species

“…In fact, even in the case of nonpyritic U ores, Fe (III) can be supplied to drive U solubilization (11). Direct anaerobic oxidation of uraninite, U(IV) to U(VI), has been observed with nitrate-reducing Thiobacillus denitrificans using c-type cytochromes (12,13). Oxidation of soluble uranous to uranyl ions coupled to CO 2 fixation has been demonstrated for A. ferrooxidans (14).…”

Uranium Bio-Transformations: Chemical or Biological Processes?

“…Typically, this happens spontaneously when oxygen invades the anaerobic environment, but there is new evidence for bacteria that oxidize uranium. To date, only two organisms have been characterized as uranium oxidizers, Geobacter metallireducens and Thiobacillusdenitrificans (Table 2) [86]. G. metallireducens is a well-known uranium reducer so it was surprising that it has also been shown to oxidize uranium; it is possible that the non-reducing cytochromes may play a role in oxidation.…”

“…G. metallireducens is a well-known uranium reducer so it was surprising that it has also been shown to oxidize uranium; it is possible that the non-reducing cytochromes may play a role in oxidation. In the nitrate-dependent uranium oxidation by β-Proteobacteria sulfur oxidizer and nitrate reducer T. denitrificans two diheme cytochrome c's accounted for at least 50% of the oxidation capacity [86]. With the addition of nitrate or active denitrification, no uranium reduction was detected suggesting nitrate amendment may stimulate uranium bio-oxidation [136].…”