Characterization of Cystathionine β-Synthase TtCbs1 and Cysteine Synthase TtCsa1 Involved in Cysteine Biosynthesis in Tetrahymena thermophila

Key substrate recognition residues in the active site of cystathionine beta-synthase from Toxoplasma gondii

Cystathionine β-synthase (CBS) catalyzes the condensation of l-serine and l-homocysteine to give l-cystathionine in the transsulfuration pathway. Recently, a few O-acetylserine (l-OAS)-dependent CBSs (OCBSs) have been found in bacteria that can exclusively function with l-OAS. CBS from Toxoplasma gondii (TgCBS) can efficiently use both l-serine and l-OAS to form l-cystathionine. In this work, a series of site-specific variants substituting S84, Y160, and Y246 with hydrophobic residues found at the same positions in OCBSs was generated to explore the roles of the hydroxyl moieties of these residues as determinants of l-serine/l-OAS preference in TgCBS. We found that the S84A/Y160F/Y246V triple mutant behaved like an OCBS in terms of both substrate requirements, showing β-replacement activity only with l-OAS, and pH optimum, which is decreased by ~1 pH unit. Formation of a stable aminoacrylate upon reaction with l-serine is prevented by the triple mutation, indicating the importance of the H-bonds between the hydroxyl groups of Y160, Y246, and S84 with l-serine in formation of the intermediate. Analysis of the independent effect of each mutation on TgCBS activity and investigation of the protein-aminoacrylate complex structure allowed for the conclusion that the hydroxyl group of Y246 has a major, but not exclusive, role in controlling the l-serine preference by efficiently stabilizing its leaving group. These studies demonstrate that differences in substrate specificity of CBSs are controlled by natural variations in as few as three residue positions. A better understanding of substrate specificity in TgCBS will facilitate the design of new antimicrobial compounds.

Identification and functional analysis of the mitochondrial cysteine synthase TtCsa2 from Tetrahymena thermophila

Cysteine is a crucial component for all organisms and plays a critical role in the structure, stability, and catalytic functions of many proteins. Tetrahymena has reverse transsulfuration and de novo pathways for cysteine biosynthesis. Cysteine synthase is involved in the de novo cysteine biosynthesis and catalyzes the production of cysteine from O-acetylserine. The novel cysteine synthase TtCSA2 was identified from Tetrahymena thermophila. The TtCSA2 showed high expression levels at the log-phase and the sexual development stage. The TtCsa2 was localized on the outer mitochondrial membrane throughout different developmental stages. However, the truncated N-terminal signal peptide mutant TtCsa2-ΔN23 was localized into the mitochondria. His-TtCsa2 was expressed in Escherichia coli and purified using affinity chromatography. The His-TtCsa2 showed O-acetylserine sulfhydrylase and serine sulfhydrylase activities. Cysteine and glutathione contents decreased in the csa2KD mutant. Furthermore, mutant cells were sensitive to cadmium and copper stresses. This study indicated that the TtCSA2 was involved in the cysteine synthesis in mitochondria and related to heavy metal stresses resistance in Tetrahymena.

Comparative transcriptome analysis uncovers roles of hydrogen sulfide for alleviating cadmium toxicity in Tetrahymena thermophila

Background

Cadmium (Cd) is a nonessential heavy metal with potentially deleterious effects on different organisms. The organisms have evolved sophisticated defense system to alleviate heavy metal toxicity. Hydrogen sulfide (H₂S) effectively alleviates heavy metal toxicity in plants and reduces oxidative stress in mammals. However, the function of H₂S for alleviating heavy metal toxicity in aquatic organisms remains less clear. Tetrahymena thermophila is an important model organism to evaluate toxic contaminants in an aquatic environment. In this study, the molecular roles of exogenously H₂S application were explored by RNA sequencing under Cd stress in T. thermophila.

Results

The exposure of 30 μM Cd resulted in T. thermophila growth inhibition, cell nigrescence, and malondialdehyde (MDA) content considerably increase. However, exogenous NaHS (donor of H₂S, 70 μM) significantly alleviated the Cd-induced toxicity by inhibiting Cd absorbtion, promoting CdS nanoparticles formation and improving antioxidant system. Comparative transcriptome analysis showed that the expression levels of 9152 genes changed under Cd stress (4658 upregulated and 4494 downregulated). However, only 1359 genes were differentially expressed with NaHS treatment under Cd stress (1087 upregulated and 272 downregulated). The functional categories of the differentially expressed genes (DEGs) by gene ontology (GO) revealed that the transcripts involved in the oxidation–reduction process, oxidoreductase activity, glutathione peroxidase activity, and cell redox homeostasis were the considerable enrichments between Cd stress and NaHS treatment under Cd stress. Kyoto Encyclopedia of Genes and Genomes (KEGG) indicated that the carbon metabolism, glutathione metabolism, metabolism of xenobiotics by cytochrome P450, and ABC transporters were significantly differentially expressed components between Cd stress and NaHS treatment under Cd stress in T. thermophila. The relative expression levels of six DEGs were further confirmed through quantitative real-time polymerase chain reaction (qRT-PCR).

Conclusion

NaHS alleviated Cd stress mainly through inhibiting Cd absorbtion, promoting CdS nanoparticles formation, increasing oxidation resistance, and regulation of transport in free-living unicellular T. thermophila. These findings will expand our understanding for H₂S functions in the freshwater protozoa.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07337-9.