The respiratory selenite reductase of the haloalkaliphile Bacillus selenitireducens MLS10
Abstract
Selenium, though toxic at elevated concentrations, is integral to life in all three domains via its incorporation into macromolecules, both in the form of organic selenide species (e.g., the 21st amino acid selenocysteine) and as an inorganic co-factor in some bacterial enzymes. The selenium oxyanions selenate (Se(VI)), selenite (Se(IV)), and elemental selenium (Se(0)) are also rich sources of energy as terminal electron acceptors for both bacteria and archaea and support a robust biogeochemical cycle. Mechanisms for Se(VI) respiration have been identified in several bacteria, suggesting it evolved after the Earth's atmosphere became substantially oxygenated. Se(IV), geochemistry suggests, would have been available to life at least as early as the late Archean. Despite this, no Se(IV) reductase in a bona fide Se(IV)-respiring organism has been described. We report the identification of the respiratory Se(IV) reductase (SrrA) from the Se(IV)- and Se(0)-respiring haloalkaliphilic bacterium Bacillus selenitireducens MLS10, isolated from the hypersaline soda lake Mono Lake in California. Our research used a polyphasic (e.g., proteomic, genomic, and biochemical) approach. In-gel enzyme assays were used to identify SrrA from periplasmic fractions, and the MLS10 genome was used to identify a 6 gene operon putatively involved in Se(IV) respiration. Phylogenetic analysis of SrrA placed the enzyme within the polysulfide/thiosulfate (Psr/Phs) reductase family of the complex iron-sulfur molybdoenzyme (CISM) superfamily. SrrA was also characterized biochemically, displaying a high affinity ( 141 uM) and specificity for Se(IV). This research emphasizes the importance of the selenium biogeochemical cycle in sustaining life in soda lake environments. It is also an integral step towards assessing the origin and evolution of Se(IV) respiration in prokaryotes, to determine if Se(IV) could have conceivably sustained life early on (i.e., prior to the divergence of bacteria and archaea), or if Se(IV) respiration evolved only after a substantial quantity of O2 enveloped the Earth surface.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B43H2954W
- Keywords:
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- 0448 Geomicrobiology;
- BIOGEOSCIENCESDE: 0456 Life in extreme environments;
- BIOGEOSCIENCESDE: 0465 Microbiology: ecology;
- physiology and genomics;
- BIOGEOSCIENCES