Mechanism of selenite reduction in Bacillus subtilis SR41: Role of thioredoxin reductase and threshold-driven transcriptomic response

Abstract

Selenium, an essential trace element for humans and animals, plays critical roles in various biological processes. However, selenite, a highly toxic form of selenium, presents substantial environmental hazards. Microbial-mediated detoxification of selenite into selenium nanoparticles (SeNPs) has emerged as an eco-friendly strategy. Bacillus subtilis SR41, a selenite-tolerant probiotic, can biosynthesize SeNPs with an average size of 67.0 ± 0.6 nm. This study is the first to report that SR41 exhibits distinct growth patterns when exposed to selenite concentrations above or below the detoxification threshold. Through a combination of in vitro assays, transcriptomics, and detection of selected-gene expression levels, thioredoxin reductase (TrxR) is identified as the primary enzyme responsible for selenite reduction, rather than glutathione reductase or fumarate reductase. The selenite reduction rate of recombinant TrxR was modeled using Michaelis-Menten kinetics, with nonlinear least squares analysis yielding a K_m value of 282.5–406.7 μM and a V_max of 332.9–373.0 μM·h⁻¹ .Notably, under selenite concentrations exceeding the detoxification threshold, TrxR is upregulated 18.21-fold, while SR41 exhibits an activation of ribosome-associated genes to mitigate the toxicity. Gene knockout experiments demonstrate the indispensability of TrxR-encoding genes. This study elucidates mechanisms of microbial selenite detoxification across concentration gradients and offers insights for environmental bioremediation applications.