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Cysteine is implicated in important biological processes. It is synthesized through two different pathways. Cystathionine β‐synthase and cystathionine γ‐lyase participate in the reverse transsulfuration pathway, while serine acetyltransferase and cysteine synthase function in the de novo pathway. Two evolutionarily related pyridoxal 5′‐phosphate‐dependent enzymes, cystathionine β‐synthase TtCBS1 (TTHERM_00558300) and cysteine synthase TtCSA1 (TTHERM_00239430), were identified from a freshwater protozoan Tetrahymena thermophila. TtCbs1 contained the N‐terminal heme binding domain, catalytic domain, and C‐terminal regulatory domain, whereas TtCsa1 consisted of two α/β domains. The catalytic core of the two enzymes is similar. TtCBS1 and TtCSA1 showed high expression levels in the vegetative growth stage and decreased during the sexual developmental stage. TtCbs1 and TtCsa1 were localized in the cytoplasm throughout different developmental stages. His‐TtCbs1 and His‐TtCsa1 were expressed and purified in vitro. TtCbs1 catalyzed the canonical reaction with the highest velocity and possessed serine sulfhydrylase activity. TtCsa1 showed cysteine synthase activity with high Km for O‐acetylserine and low Km for sulfide and also had serine sulfhydrylase activity toward serine. Both TtCbs1 and TtCsa1 catalyzed hydrogen sulfide producing. TtCBS1 knockdown and TtCSA1 knockout mutants affected cysteine and glutathione synthesis. TtCbs1 and TtCsa1 are involved in cysteine synthesis through two different pathways in T. 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.

BackgroundCadmium (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. 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.ResultsThe exposure of 30 µM Cd resulted in T. thermophila growth inhibition, cell nigrescence, and malondialdehyde (MDA) content considerably increase. However, exogenous H2S (70 µM) significantly alleviated the Cd-induced toxicity and improved 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 H2S 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 H2S 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 H2S 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).ConclusionH2S alleviated Cd stress mainly through increasing oxidation resistance, enhancing detoxification, and regulation of transport in free-living unicellular T. thermophila. These findings will expand our understanding for H2S functions in the freshwater protozoa.

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.

Early events in the evolution of an ancestral lineage can shape the adaptive patterns of descendant species, but the evolutionary mechanisms driving initial adaptation from an ancestor remain largely unexplored. High‐altitude adaptations have been extensively explored from the viewpoint of protein‐coding genes; however, the contribution of noncoding regions remains relatively neglected. Here, we integrate genomic and transcriptomic data to investigate adaptive evolution in the ancestor of three high‐altitude snowfinch species endemic to the Qinghai–Tibet Plateau. Our genome‐wide scan for adaptation in the snowfinch ancestor identifies strong adaptation signals in functions of development and metabolism for the coding genes, but in functions of the nervous system development for noncoding regions. This pattern is exclusive to the snowfinch ancestor compared to a control ancestral lineage subject to weak selection. Changes in noncoding regions in the snowfinch ancestor, especially those nearest to coding genes, may be disproportionately associated with the differential expression of genes in the brain tissue compared to other tissues. Extensive gene expression in the brain tissue can be further altered via genetic regulatory networks of transcription factors harbouring potential accelerated regulatory regions (e.g., the development‐related transcription factor YEATS4). Altogether, our study provides new evidence concerning how coding and noncoding sequences work through decoupled pathways in initial adaptation to the selective pressure of high‐altitude environments. The analysis highlights the idea that noncoding sequences may be promising elements in facilitating the rapid evolution and adaptation to high altitudes.

Gut microbial communities of animals play key roles in host evolution, while the relationship between gut microbiota and host evolution in Tibetan birds remains unknown. Herein, we sequenced the gut microbiota of 67 wild birds of seven species dwelling in the Tibetan wetlands. We found an obvious species-specific structure of gut microbiota among these plateau birds whose habitats were overlapped. Different from plateau mammals, there was no strict synergy between the hierarchical tree of gut microbial community and species phylogeny. In brown-headed gulls (Larus brunnicephalus) as an example, the structure of gut microbiota differed in different habitats, and the relative abundance of bacteria, such as Lactobacillus, Streptococcus, Paracoccus, Lachnospiraceae, and Vibrio, significantly correlated with altitude. Finally, we found various pathogenic bacteria in the birds of these plateau wetlands, and the interspecific differences were related to their diet and living environments.

A new species of the genus Synechococcus C. Nägeli was described from extreme environment (high salinity) of the Yuncheng salt lake, North China. Morphological characteristics observed by light microscopy (LM) and transmission electron microscopy (TEM) were described. DNA barcodes (16S rRNA+ITS-1, cpcBA-IGS) were used to evaluate its taxonomic status. This species was identified as Synechococcus salsus H. Lv et S. Xie. It is characterized by unicellular, without common mucilage, cells with several dispersed or solitary polyhedral bodies, widely coccoid, sometimes curved or sigmoid, rounded at the ends, thylakoids localized along cells walls. Molecular analyses further support its systematic position as an independent branch. The new species Synechococcus salsus is closely allied to S. elongatus, C. Nägeli, but differs from it by having shorter cell with length 1.0-1.5 times of width.