Thermostable and highly specific L-aspartate oxidase from Thermococcus litoralis DSM 5473: cloning, overexpression, and enzymological properties

Transcription factor AbrB regulates ROS generation and clearance in Bacillus licheniformis

Oxidative damage caused by the accumulation of reactive oxygen species (ROS) is one of the main obstacles to the improvement of microbial cell growth and fermentation characteristics under adverse environments. And the antioxidant capacity of cells will increase with the cell growth. Here, we found that a transition state transcription factor AbrB related to changes in cell growth status could regulate the accumulation of ROS and antioxidant capacity in Bacillus licheniformis. The results showed that the accumulation of intracellular ROS was reduced by 23.91 % and the cell survival rates were increased by 1.77-fold under 0.5 mM H2O2 when AbrB was knocked out. We further mapped regulatory target genes of AbrB related to ROS generation or clearance based on our previously analyzed transcriptome sequencing. It proved that AbrB could promote ROS generation via upregulating the synthesis of oxidase and siderophores, and negatively regulating the synthesis of iron chelators (pulcherriminic acid, and H2S). Additionally, AbrB could inhibit ROS clearance by negatively regulating the synthesis of antioxidase (superoxide dismutase, catalase, peroxidase, thioredoxin, thioredoxin reductase) and cysteine. Those results illustrated that the inactivation of AbrB during the stationary phase, along with its control over ROS generation and clearance, might represent a vital self-protection mechanism during cell evolution. Overall, the systematic investigation of the multi-pathway regulation network of ROS generation and clearance highlights the important function of AbrB in maintaining intracellular redox balance.

Regulation of L-aspartate oxidase contributes to NADP+ biosynthesis in Synechocystis sp. PCC 6803

Cyanobacteria have been promoted as a biomass resource that can contribute to carbon neutrality. Synechocystis sp. PCC 6803 is a model cyanobacterium that is widely used in various studies. NADP+ and NAD+ are electron receptors involved in energy metabolism. The NADP+/NAD+ ratio in Synechocystis sp. PCC 6803 is markedly higher than that in the heterotrophic bacterium Escherichia coli. In Synechocystis sp. PCC 6803, NADP+ primarily functions as an electron receptor during the light reaction of photosynthesis, and NADP+ biosynthesis is essential for photoautotrophic growth. Generally, the regulatory enzyme of NADP+ biosynthesis is NAD kinase, which catalyzes the phosphorylation of NAD+. However, a previous study suggested that the regulation of another enzyme contributes to NADP+ biosynthesis in Synechocystis sp. PCC 6803 under photoautotrophic conditions. L-Aspartate oxidase is the first enzyme in NAD(P)+ biosynthesis. In this study, we biochemically characterized Synechocystis sp. PCC 6803 L-aspartate oxidase and determined the phenotype of a Synechocystis sp. PCC 6803 mutant overexpressing L-aspartate oxidase. The catalytic efficiency of L-aspartate oxidase from Synechocystis sp. PCC 6803 was lower than that of L-aspartate oxidases and NAD kinases from other organisms. L-Aspartate oxidase activity was affected by different metabolites such as NADP+ and ATP. The L-aspartate oxidase-overexpressing strain grew faster than the wild-type strain under photoautotrophic conditions. The L-aspartate oxidase-overexpressing strain accumulated NADP+ under photoautotrophic conditions. These results indicate that the regulation of L-aspartate oxidase contributes to NADP+ biosynthesis in Synechocystis sp. PCC 6803 under photoautotrophic conditions. These findings provide insight into the regulatory mechanism of cyanobacterial NADP+ biosynthesis.

Hyperthermophilic Archaeon Thermococcus kodakarensis Utilizes a Four-Step Pathway for NAD+ Salvage through Nicotinamide Deamination

Many organisms possess pathways that regenerate NAD⁺ from its degradation products, and two pathways are known to salvage NAD⁺ from nicotinamide (Nm). One is a four-step pathway that proceeds through deamination of Nm to nicotinic acid (Na) by Nm deamidase and phosphoribosylation to nicotinic acid mononucleotide (NaMN), followed by adenylylation and amidation. Another is a two-step pathway that does not involve deamination and directly proceeds with the phosphoribosylation of Nm to nicotinamide mononucleotide (NMN), followed by adenylylation. Judging from genome sequence data, the hyperthermophilic archaeon Thermococcus kodakarensis is supposed to utilize the four-step pathway, but the fact that the adenylyltransferase encoded by TK0067 recognizes both NMN and NaMN also raises the possibility of a two-step salvage mechanism. Here, we examined the substrate specificity of the recombinant TK1676 protein, annotated as nicotinic acid phosphoribosyltransferase. The TK1676 protein displayed significant activity toward Na and phosphoribosyl pyrophosphate (PRPP) and only trace activity with Nm and PRPP. We further performed genetic analyses on TK0218 (quinolinic acid phosphoribosyltransferase) and TK1650 (Nm deamidase), involved in de novo biosynthesis and four-step salvage of NAD⁺, respectively. The ΔTK0218 mutant cells displayed growth defects in a minimal synthetic medium, but growth was fully restored with the addition of Na or Nm. The ΔTK0218 ΔTK1650 mutant cells did not display growth in the minimal medium, and growth was restored with the addition of Na but not Nm. The enzymatic and genetic analyses strongly suggest that NAD⁺ salvage in T. kodakarensis requires deamination of Nm and proceeds through the four-step pathway.IMPORTANCE Hyperthermophiles must constantly deal with increased degradation rates of their biomolecules due to their high growth temperatures. Here, we identified the pathway that regenerates NAD⁺ from nicotinamide (Nm) in the hyperthermophilic archaeon Thermococcus kodakarensis The organism utilizes a four-step pathway that initially hydrolyzes the amide bond of Nm to generate nicotinic acid (Na), followed by phosphoribosylation, adenylylation, and amidation. Although the two-step pathway, consisting of only phosphoribosylation of Nm and adenylylation, seems to be more efficient, Nm mononucleotide in the two-step pathway is much more thermolabile than Na mononucleotide, the corresponding intermediate in the four-step pathway. Although NAD⁺ itself is thermolabile, this may represent an example of a metabolism that has evolved to avoid the use of thermolabile intermediates.