Respiratory Selenite Reductase from Bacillus selenitireducens Strain MLS10

Area of Science:

• Microbiology
• Biochemistry
• Environmental Science

Background:

• Selenium is an essential element, and selenite [Se(IV)] reduction is vital for its biogeochemical cycling.
• Dissimilatory Se(IV) reduction is a key microbial process in selenium transformation.
• Understanding the enzymes involved in selenium metabolism is crucial for environmental and biological studies.

Purpose of the Study:

• To identify and characterize the putative respiratory selenite [Se(IV)] reductase (Srr) from *Bacillus selenitireducens* MLS10.
• To elucidate the genetic organization and protein components of the Srr enzyme complex.
• To determine the substrate specificity and kinetic properties of the identified Srr.

Main Methods:

• Polyphasic approach combining genomics, proteomics, and enzymology.
• Nondenaturing gel electrophoresis and protein staining to identify active enzyme fractions.
• Liquid chromatography-tandem mass spectrometry (LC-MS/MS) for protein identification.
• Enzyme assays to determine substrate specificity and kinetic parameters (Km, Vmax, kcat).

Main Results:

• The respiratory selenite [Se(IV)] reductase (Srr) was identified as an 80 kDa protein, homologous to the catalytic subunit of polysulfide reductase (PsrA).
• The Srr enzyme is encoded by a six-gene operon (*srrE, srrA, srrB, srrC, srrD, srrF*), with SrrA as the catalytic subunit.
• Phylogenetic analysis places SrrA within the PsrA/PhsA clade, and the enzyme exhibits specificity for Se(IV) with a Km of 145 ± 53 μM.

Conclusions:

• This study reports the first identification of a dissimilatory Se(IV) reductase (Srr) from *Bacillus selenitireducens* MLS10.
• The findings expand the known functions of the complex iron-sulfur molybdoenzyme (CISM) superfamily in electron transfer with chalcogen substrates.
• Biochemical and enzymological characterization is essential for understanding the functionality of enzymes involved in selenium biotransformation and the biogeochemical cycle.