Cloning of Nitrate Reductase and Nitrite Reductase Genes and Their Functional Analysis in Regulating Cr(VI) Reduction in Ectomycorrhizal Fungus Pisolithus sp.1

Heterologous expression, enzymatic properties, product analysis and molecular docking of assimilative nitrite reductase (NiR) in Bacillus velezensis GXMZU-B1 derived from mariculture

High concentrations of nitrite stress aquatic animals, leading to significant fish and shrimp deaths as well as environmental pollution. Reducing nitrite levels in high-density aquaculture is crucial for both aquaculture safety and environmental protection. Nitrite reductase (NiR) can rapidly reduce nitrite in water, offering potential applications in aquaculture and water treatment. In this study, a novel NiR gene (nasD) was isolated from Bacillus velezensis GXMZU-B1, a highly effective nitrite-degrading bacterium, and expressed heterologously in Escherichia coli. The recombinant NASD was purified using Ni-NTA affinity chromatography, and its physicochemical properties and reaction products were analyzed. The enzyme showed optimal activity at 30°C and pH 6.5. Metal ions such as Fe3+, Zn2+, and Ba2+ enhanced enzyme activity, whereas Cu2+, K+, Mg2+, and Mn2+ reduced it. The best electron donors was NADPH. NASD converts nitrite (NO2-) into ammonium (NH4+), making it environmentally friendly and potentially valuable for aquaculture and water pollution control. Bioinformatics analysis indicated that the enzyme is stable, with a conserved sequence and a Pyr_redox_2 domain. Using NADPH as a coenzyme, AlphaFold3 modeling and molecular docking with nitrite identified 14 potential catalytic sites. These findings highlight the potential of recombinant NASD as a promising candidate for nitrite degradation in aquaculture.

Recovery mechanism of bio-promoters on Cr(VI) suppressed denitrification: Toxicity remediation and enhanced electron transmission

Although the biotoxicity of heavy metals has been widely studied, there are few reports on the recovery strategy of the inhibited bio-system. This study proposed a combined promoter-I (Primary promoter: l-cysteine, biotin, and cytokinin + Electron-shuttle: PMo12) to recover the denitrification suppressed by Cr(VI). Compared with self-recovery, combined promoter-I shortened the recovery time of 28 cycles, and the recovered reactor possessed more stable long-term operation performance with >95 % nitrogen removal. The biomass increased by 7.07 mg VSS/(cm3 carrier) than self-recovery due to the promoted bacterial reproduction, thereby reducing the toxicity load of chromium per unit biomass. The combined promoter-I strengthened the toxicity remediation by promoting 92.84 % of the intracellular chromium release and rapidly activating anti-oxidative stress response. During toxicity remediation, ROS content quickly decreased, and the PN/PS value was 2.27 times that of self-recovery. PMo12 relieved Cr(VI) inhibition on NO3--N reduction by increasing NAR activity. The enhanced intracellular and intercellular electron transmission benefited from the stimulated NADH, FMN, and Cyt.c secretion by the primary promoter and the improved transmembrane electron transmission by Mo. PMo12 and the primary promoter synergized in regulating community structure and improving microbial richness. This study provided practical approaches for microbial toxicity remediation and maintaining high-efficiency denitrification.

Metabolic mechanism of Cr(VI) pollution remediation by Alicycliphilus denitrificans Ylb10

Cr(VI) is a well-known toxic pollutant and its remediation has attracted great attention. It is important to continuously discover and explore new high-efficiency Cr(VI) reducing bacteria to further improve the efficiency of Cr(VI) pollution remediation. In this paper, metabolic mechanism of Cr(VI) reduction in a new highly efficient Cr(VI) reducing bacterium, Alicycliphilus denitrificans Ylb10, was investigated. The results showed that Ylb10 could tolerate and completely reduce 450 mg/L Cr(VI). Cr(VI) can be reduced in the intracellular compartment, membrane and the extracellular compartment, with the plasma membrane being the main active site for Cr(VI) reduction. With the addition of NADH, the reduction efficiency of cell membrane components for Cr(VI) increased 2.3-fold. The omics data analysis showed that sulfite reductase CysJ, thiosulfate dehydrogenase TsdA, nitrite reductase NrfA, nitric oxide reductase NorB, and quinone oxidoreductase ChrR play important roles in the reduction of Cr(VI), in the intracellular, and the extracellular compartment, and the membrane of Ylb10, and therefore Cr(VI) was reduced by the combined action of several reductases at these three locations.

Nitric Oxide in Fungi: Production and Function

Nitric oxide (NO) is synthesized in all kingdoms of life, where it plays a role in the regulation of various physiological and developmental processes. In terms of endogenous NO biology, fungi have been less well researched than mammals, plants, and bacteria. In this review, we summarize and discuss the studies to date on intracellular NO biosynthesis and function in fungi. Two mechanisms for NO biosynthesis, NO synthase (NOS)-mediated arginine oxidation and nitrate- and nitrite-reductase-mediated nitrite reduction, are the most frequently reported. Furthermore, we summarize the multifaceted functions of NO in fungi as well as its role as a signaling molecule in fungal growth regulation, development, abiotic stress, virulence regulation, and metabolism. Finally, we present potential directions for future research on fungal NO biology.