Short-chain dehydrogenases/reductases in cyanobacteria

Whole genome-scale assessment of gene fitness of Novosphingobium aromaticavorans during spaceflight

In microgravity, bacteria undergo intriguing physiological adaptations. There have been few attempts to assess global bacterial physiological responses to microgravity, with most studies only focusing on a handful of individual systems. This study assessed the fitness of each gene in the genome of the aromatic compound-degrading Alphaproteobacterium Novosphingobium aromaticavorans during growth in spaceflight. This was accomplished using Comparative TnSeq, which involves culturing the same saturating transposon mutagenized library under two different conditions. To assess gene fitness, a novel comparative TnSeq analytical tool was developed, named TnDivA, that is particularly useful in leveraging biological replicates. In this approach, transposon diversity is represented numerically using a modified Shannon diversity index, which was then converted into effective transposon density. This transformation accounts for variability in read distribution between samples, such as cases where reads were dominated by only a few transposon inserts. Effective density values were analyzed using multiple statistical methods, including log2-fold change, least-squares regression analysis, and Welch's t-test. The results obtained across applied statistical methods show a difference in the number of significant genes identified. However, the functional categories of genes important to growth in microgravity showed similar patterns. Lipid metabolism and transport, energy production, transcription, translation, and secondary metabolite biosynthesis and transport were shown to have high fitness during spaceflight. This suggests that core metabolic processes, including lipid and secondary metabolism, play an important role adapting to stress and promoting growth in microgravity.

Expression, Purification, and Bioinformatic Prediction of Mycobacterium tuberculosis Rv0439c as a Potential NADP+-Retinol Dehydrogenase

Although the genome of Mycobacterium tuberculosis (Mtb) H37Rv, the causative agent of tuberculosis, has been repeatedly annotated and updated, a range of proteins from this human pathogen have unknown functions. Mtb Rv0439c, a member of the short-chain dehydrogenase/reductases superfamily, has yet to be cloned and characterized, and its function remains unclear. In this work, we present for the first time the optimized expression and purification of this enzyme, as well as bioinformatic analysis to unveil its potential coenzyme and substrate. Optimized expression in Escherichia coli yielded soluble Rv0439c, while certain tag fusions resulted in insolubility. Sequence and docking analyses strongly suggested that Rv0439c has a clear preference for NADP+, with Arg53 being a key residue that confers coenzyme specificity. Furthermore, functional prediction using CLEAN and DEEPre servers suggested that this protein is a potential NADP+-retinol dehydrogenase (EC No. 1.1.1.300) in retinol metabolism, and this was supported by a BLASTp search and docking studies. Collectively, our findings provide a solid basis for future functional characterization and structural studies of Rv0439c, which will contribute to enhanced understanding of Mtb biology.

Function discovery of a non-ribosomal peptide synthetase-like encoding gene in the nematode-trapping fungus Arthrobotrys oligospora

In this study, the function of a non-ribosomal peptide synthetase-like (NRPS-like) encoding gene AOL_s00188g306 (g306) was investigated to reveal the association between NRPS and nematocidal activity in the nematode-trapping fungus Arthrobotrys oligospora. Sequence analysis indicated that the encoded product of g306 is an adenylation domain of non-ribosomal peptide synthetases and extended short-chain dehydrogenase/reductase domain-containing proteins, and displays a wide substrate spectrum. The Δg306 mutants were more sensitive to chemical stressors than the wild type. Disruption of g306 impeded the nematocidal efficiency of A. oligospora. Metabolomics analysis showed that secondary metabolite biosynthesis and lipid metabolism were altered in the mutants. The phenotypic changes in the mutants can be attributed to the down-regulation of various metabolites, including fatty acyls, prenol lipids, steroidsand steroid derivative, and amino acid derivatives, identified in the present study. This study investigated the association between the non-ribosomal polypeptide-encoding gene g306 and nematicidal activity in A. oligospora, providing a reference for resolving the predation mechanism of nematode-trapping fungus.

Bioinformatic analysis of short-chain dehydrogenase/reductase proteins in plant peroxisomes

Peroxisomes are ubiquitous eukaryotic organelles housing not only many important oxidative metabolic reactions, but also some reductive reactions that are less known. Members of the short-chain dehydrogenase/reductase (SDR) superfamily, which are NAD(P)(H)-dependent oxidoreductases, play important roles in plant peroxisomes, including the conversion of indole-3-butyric acid (IBA) to indole-3-acetic acid (IAA), auxiliary β-oxidation of fatty acids, and benzaldehyde production. To further explore the function of this family of proteins in the plant peroxisome, we performed an in silico search for peroxisomal SDR proteins from Arabidopsis based on the presence of peroxisome targeting signal peptides. A total of 11 proteins were discovered, among which four were experimentally confirmed to be peroxisomal in this study. Phylogenetic analyses showed the presence of peroxisomal SDR proteins in diverse plant species, indicating the functional conservation of this protein family in peroxisomal metabolism. Knowledge about the known peroxisomal SDRs from other species also allowed us to predict the function of plant SDR proteins within the same subgroup. Furthermore, in silico gene expression profiling revealed strong expression of most SDR genes in floral tissues and during seed germination, suggesting their involvement in reproduction and seed development. Finally, we explored the function of SDRj, a member of a novel subgroup of peroxisomal SDR proteins, by generating and analyzing CRISPR/Cas mutant lines. This work provides a foundation for future research on the biological activities of peroxisomal SDRs to fully understand the redox control of peroxisome functions.

Differential analysis of transcriptomic and metabolomic of free fatty acid rancidity process in oil palm (Elaeis guineensis) fruits of different husk types

INTRODUCTION

Oil palm is the world's highest yielding oil crop and its palm oil has high nutritional value, making it an oilseed plant with important economic value and application prospects. After picking, oil palm fruits exposed to air will gradually become soft and accelerate the process of fatty acid rancidity, which will not only affect their flavor and nutritional value, but also produce substances harmful to the human body. As a result, studying the dynamic change pattern of free fatty acids and important fatty acid metabolism-related regulatory genes during oil palm fatty acid rancidity can provide a theoretical basis for improving palm oil quality and extending its shelf life.

METHODS

The fruit of two shell types of oil palm, Pisifera (MP) and Tenera (MT), were used to study the changes of fruit souring at different times points of postharvesting, combined with LC-MS/MS metabolomics and RNA-seq transcriptomics techniques to analyze the dynamic changes of free fatty acids during fruit rancidity, and to find out the key enzyme genes and proteins in the process of free fatty acid synthesis and degradation according to metabolic pathways.

RESULTS AND DISCUSSION

Metabolomic study revealed that there were 9 different types of free fatty acids at 0 hours of postharvest, 12 different types of free fatty acids at 24 hours of postharvest, and 8 different types of free fatty acids at 36 hours of postharvest. Transcriptomic research revealed substantial changes in gene expression between the three harvest phases of MT and MP. Combined metabolomics and transcriptomics analysis results show that the expression of SDR, FATA, FATB and MFP four key enzyme genes and enzyme proteins in the rancidity of free fatty acids are significantly correlated with Palmitic acid, Stearic acid, Myristic acid and Palmitoleic acid in oil palm fruit. In terms of binding gene expression, the expression of FATA gene and MFP protein in MT and MP was consistent, and both were expressed higher in MP. FATB fluctuates unevenly in MT and MP, with the level of expression growing steadily in MT and decreasing in MP before increasing. The amount of SDR gene expression varies in opposite directions in both shell types. The above findings suggest that these four enzyme genes and enzyme proteins may play an important role in regulating fatty acid rancidity and are the key enzyme genes and enzyme proteins that cause differences in fatty acid rancidity between MT and MP and other fruit shell types. Additionally, differential metabolite and differentially expressed genes were present in the three postharvest times of MT and MP fruits, with the difference occurring 24 hours postharvest being the most notable. As a result, 24 hours postharvest revealed the most obvious difference in fatty acid tranquility between MT and MP shell types of oil palm. The results from this study offer a theoretical underpinning for the gene mining of fatty acid rancidity of various oil palm fruit shell types and the enhancement of oilseed palm acid-resistant germplasm cultivation using molecular biology methods.

Nitric oxide enhances resistance of Pleurotus eryngii to cadmium stress by alleviating oxidative damage and regulating of short-chain dehydrogenase/reductase family

The function and mechanism of nitric oxide (NO) in regulating Pleurotus eryngii biological response to cadmium (Cd) stress was evaluated by using anti-oxidation and short-chain dehydrogenase/reductase (SDR) family analysis. The fresh biomass of P. eryngii mycelia sharply decreased after treatment with 50 µM Cd; the lipid peroxidation and H₂O₂ accumulation in P. eryngii were found responsible for it. Proper exogenous supply of NO (150 µM SNP) alleviated the oxidative damage induced by Cd stress in P. eryngii, which reduced the accumulation of thiobarbituric acid reactive substances (TBARS) and H₂O₂. The activities of antioxidant enzymes (superoxide dismutase, peroxidase) were significantly increased to deal with Cd stress when treated with SNP (150 µM), and the content of proline was also closely related to NO-mediated reduction of Cd toxicity. Moreover, SDR family members were widely involved in the response to Cd stress, especially PleSCH70 gene was observed for the first time in participating in NO-mediated enhancement of Cd tolerance in P. eryngii. Taken together, this study provides new insights in understanding the tolerance mechanisms of P. eryngii to heavy metal and lays a foundation for molecular breeding of P. eryngii to improve its tolerance to environmental stress.

Genomic analysis of Gordonia polyisoprenivorans strain R9, a highly effective 17 beta-estradiol- and steroid-degrading bacterium

The increasing levels of estrogens and pollution by other steroids pose considerable challenges to the environment. In this study, the genome of Gordonia polyisoprenivorans strain R9, one of the most effective 17 beta-estradiol- and steroid-degrading bacteria, was sequenced and annotated. The circular chromosome of G. polyisoprenivorans R9 was 6,033,879 bp in size, with an average GC content of 66.91%. More so, 5213 putative protein-coding sequences, 9 rRNA, 49 tRNA, and 3 sRNA genes were predicted. The core-pan gene evolutionary tree for the genus Gordonia showed that G. polyisoprenivorans R9 is clustered with G. polyisoprenivorans VH2 and G. polyisoprenivorans C, with 93.75% and 93.8% similarity to these two strains, respectively. Altogether, the three G. polyisoprenivorans strains contained 3890 core gene clusters. Strain R9 contained 785 specific gene clusters, while 501 and 474 specific gene clusters were identified in strains VH2 and C, respectively. Furthermore, whole genome analysis revealed the existence of the steroids and estrogens degradation pathway in the core genome of all three G. polyisoprenivorans strains, although the G. polyisoprenivorans R9 genome contained more specific estrogen and steroid degradation genes. In strain R9, 207 ABC transporters, 95 short-chain dehydrogenases (SDRs), 26 monooxygenases, 21 dioxygenases, 7 aromatic ring-hydroxylating dioxygenases, and 3 CoA esters were identified, and these are very important for estrogen and steroid transport, and degradation. The results of this study could enhance our understanding of the role of G. polyisoprenivorans R9 in estradiol and steroid degradation as well as evolution within the G. polyisoprenivorans species.

Identification, Expression and Evolution of Short-Chain Dehydrogenases/Reductases in Nile Tilapia (Oreochromis niloticus)

The short-chain dehydrogenases/reductases (SDR) superfamily is involved in multiple physiological processes. In this study, genome-wide identification and comprehensive analysis of SDR superfamily were carried out in 29 animal species based on the latest genome databases. Overall, the number of SDR genes in animals increased with whole genome duplication (WGD), suggesting the expansion of SDRs during evolution, especially in 3R-WGD and polyploidization of teleosts. Phylogenetic analysis indicated that vertebrates SDRs were clustered into five categories: classical, extended, undefined, atypical, and complex. Moreover, tandem duplication of hpgd-a, rdh8b and dhrs13 was observed in teleosts analyzed. Additionally, tandem duplications of dhrs11-a, dhrs7a, hsd11b1b, and cbr1-a were observed in all cichlids analyzed, and tandem duplication of rdh10-b was observed in tilapiines. Transcriptome analysis of adult fish revealed that 93 SDRs were expressed in more than one tissue and 5 in one tissue only. Transcriptome analysis of gonads from different developmental stages showed that expression of 17 SDRs were sexually dimorphic with 11 higher in ovary and 6 higher in testis. The sexually dimorphic expressions of these SDRs were confirmed by in situ hybridization (ISH) and qPCR, indicating their possible roles in steroidogenesis and gonadal differentiation. Taken together, the identification and the expression data obtained in this study contribute to a better understanding of SDR superfamily evolution and functions in teleosts.

Crystal structures of cyanobacterial light-dependent protochlorophyllide oxidoreductase

The reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) is the penultimate step of chlorophyll biosynthesis. In oxygenic photosynthetic bacteria, algae, and plants, this reaction can be catalyzed by the light-dependent Pchlide oxidoreductase (LPOR), a member of the short-chain dehydrogenase superfamily sharing a conserved Rossmann fold for NAD(P)H binding and the catalytic activity. Whereas modeling and simulation approaches have been used to study the catalytic mechanism of this light-driven reaction, key details of the LPOR structure remain unclear. We determined the crystal structures of LPOR from two cyanobacteria, Synechocystis sp. PCC 6803 and Thermosynechococcus elongatus Structural analysis defines the LPOR core fold, outlines the LPOR-NADPH interaction network, identifies the residues forming the substrate cavity and the proton-relay path, and reveals the role of the LPOR-specific loop. These findings provide a basis for understanding the structure-function relationships of the light-driven Pchlide reduction.

Augmenting Fremyella diplosiphon Cellular Lipid Content and Unsaturated Fatty Acid Methyl Esters Via Sterol Desaturase Gene Overexpression

Cyanobacteria have immense prospective as a platform for renewable energy; however, a major barrier in achieving optimal productivity is the low lipid yield. Fremyella diplosiphon, a model cyanobacterium, is an ideal biofuel agent due to its desirable fatty acid methyl esters (FAMEs). To enhance lipid content, we overexpressed the sterol desaturase (SD) gene in F. diplosiphon B481 wild type by genetic transformation. This effort resulted in a transformant (B481-SD) with a 64-fold increase in the SD gene at the mRNA transcript level, with no loss in growth and pigmentation. The transformant was persistently grown for over 32 generations indicating long-term stability and vitality. We observed 27.3% and 23% increases in total lipid content and unsaturated FAMEs respectively in B481-SD transesterified lipids with methyl octadecadienoate as the most abundant unsaturated component. In addition, we detected an 81% increase in FAME composition in the transformant compared with the wild type. Theoretical physical and chemical properties confirmed a FAME profile with very high cetane number (65.972-67.494) and oxidative stability (50.493-18.66 h) in the engineered strain. Results of the study offer a promising approach to augment F. diplosiphon total lipid content and unsaturated FAMEs, thus paving the way to enhance biofuel capacity of the organism.

Structural and functional investigation of AerF, a NADPH-dependent alkenal double bond reductase participating in the biosynthesis of Choi moiety of aeruginosin

The 2-carboxy-6-hydroxyoctahydroindole (Choi) moiety is an essential residue for the antithrombotic activities of aeruginosins, which are a class of cyanobacterial derived bioactive linear tetrapeptides. Biosynthetic pathway of Choi is still elusive. AerF was suggested to be involved in the biosynthesis of Choi, and can be assigned to the short-chain dehydrogenase/reductase (SDR) superfamily. However, both the exact role and the catalytic mechanism of AerF have not been elucidated. In this study, functional and mechanistic analyses of AerF from Microcystis aeruginosa were performed. Observation of enzymatic assay demonstrates that AerF is a NADPH-dependent alkenal double bond reductase that catalyzes the reduction of dihydro-4-hydroxyphenylpyruvate (H₂HPP) to generate tetrahydro-4-hydroxyphenylpyruvate (H₄HPP), which is the third step of the biosynthetic pathway from prephenate to Choi. Comparative structural analysis indicates that ligand binding-induced conformational change of AerF is different from that of the other SDR superfamily reductase using H₂HPP as a substrate. Analyses of NADPH and substrate analogue binding sites combined with the results of mutagenesis analyses suggest that a particular serine residue mainly involves in the initiation of the proton transfer between the substrate and the residues of AerF, which is an uncommon feature in SDR superfamily reductase. Furthermore, based on the observations of structural and mutagenesis analyses, the catalytic mechanism of AerF is proposed and a proton transfer pathway in AerF is deduced.

Rdh13 deficiency weakens carbon tetrachloride-induced liver injury by regulating Spot14 and Cyp2e1 expression levels

Mitochondrion-localized retinol dehydrogenase 13 (Rdh13) is a short-chain dehydrogenase/reductase involved in vitamin A metabolism in both humans and mice. We previously generated Rdh13 knockout mice and showed that Rdh13 deficiency causes severe acute retinal light damage. In this study, considering that Rdh13 is highly expressed in mouse liver, we further evaluated the potential effect of Rdh13 on liver injury induced by carbon tetrachloride (CCl₄). Although Rdh13 deficiency showed no significant effect on liver histology and physiological functions under regular culture, the Rdh13^-/- mice displayed an attenuated response to CCl₄-induced liver injury. Their livers also exhibited less histological changes and contained lower levels of liver-related metabolism enzymes compared with the livers of wild-type (WT) mice. Furthermore, the Rdh13^-/- mice had Rdh13 deficiency and thus their liver cells were protected from apoptosis, and the quantity of their proliferative cells became lower than that in WTafter CCl₄ exposure. The ablation of Rdh13 gene decreased the expression levels of thyroid hormone-inducible nuclear protein 14 (Spot14) and cytochrome P450 (Cyp2e1) in the liver, especially after CCl₄ treatment for 48 h. These data suggested that the alleviated liver damage induced by CCl₄ in Rdh13^-/- mice was caused by Cyp2e1 enzymes, which promoted reductive CCl₄ metabolism by altering the status of thyroxine metabolism. This result further implicated Rdh13 as a potential drug target in preventing chemically induced liver injury.

Characterization of a Prenyltransferase for Iso-A82775C Biosynthesis and Generation of New Congeners of Chloropestolides

Chloropupukeananin and chloropestolides are novel metabolites of the plant endophyte Pestalotiopsis fici, showing antimicrobial, antitumor, and anti-HIV activities. Their highly complex and unique skeletons were generated from the coisolated pestheic acid (1) and iso-A82775C (10) based on our previous studies. Here, we identified the biosynthetic gene cluster iac of 10 and characterized an iacE encoded prenyltransferase. Deletion of iacE abolished iso-A82775C production, accumulated the prenyl group-lacking siccayne (2), and generated four new chloropestolides (3-6). Compounds 5 and 6 showed antibacterial effects against Staphylococcus aureus and Bacillus subtilis, and 5 was also cytotoxic to human tumor cell lines HeLa, MCF-7, and SW480. These results provided the first genetic and biochemical insights into the biosynthesis of natural prenylepoxycyclohexanes and demonstrated the feasibility for generation of diversified congeners by manipulating the biosynthetic genes of 10.

Ethanol, glycogen and glucosylglycerol represent competing carbon pools in ethanol-producing cells of Synechocystis sp. PCC 6803 under high-salt conditions

Cyanobacteria are photoautotrophic micro-organisms, which are increasingly being used as microbial cell factories to produce, for example, ethanol directly from solar energy and CO2. Here, we analysed the effects of different salt concentrations on an ethanol-producing strain of Synechocystis sp. PCC 6803 that overexpresses the pyruvate decarboxylase (pdc) from Zymomonas mobilis and the native alcohol dehydrogenase (adhA). Moderate salinities of 2 % NaCl had no negative impact on ethanol production, whereas the addition of 4 % NaCl resulted in significantly decreased ethanol yields compared to low-salt conditions. Proteomic analysis identified a defined set of proteins with increased abundances in ethanol-producing cells. Among them, we found strong up-regulation of α-1,4 glucan phosphorylase (GlgP, Slr1367) in the producer strain, which consistently resulted in a massive depletion of glycogen pools in these cells regardless of the salinity. The salt-induced accumulation of the compatible solute glucosylglycerol was not affected by the ethanol production. Glycogen and probably compatible solutes could present competing pools with respect to organic carbon, explaining the decreased ethanol production at the highest salinity.

Thylakoid Development and Galactolipid Synthesis in Cyanobacteria

Cyanobacteria carry out oxygenic photosynthesis and share many features with chloroplasts, including thylakoid membranes, which are mainly composed of membrane lipids and protein complexes that mediate photosynthetic electron transport. Although the functions of the various thylakoid protein complexes have been well characterized, the details underlying the biogenesis of thylakoid membranes remain unclear. Galactolipids are the major constituents of the thylakoid membrane system, and all the genes involved in galactolipid biosynthesis were recently identified. In this chapter, I summarize recent advances in our understanding of the factors involved in thylakoid development, including regulatory proteins and enzymes that mediate lipid biosynthesis.

Diversity in Biosynthetic Pathways of Galactolipids in the Light of Endosymbiotic Origin of Chloroplasts

Cyanobacteria and chloroplasts perform oxygenic photosynthesis, and share a common origin. Galactolipids are present in the photosynthetic membranes of both cyanobacteria and chloroplasts, but the biosynthetic pathways of the galactolipids are significantly different in the two systems. In this minireview, we explain the history of the discovery of the cyanobacterial pathway, and present a probable scenario of the evolution of the two pathways.

Quantitative proteomics analysis of an ethanol- and a lactate-producing mutant strain of Synechocystis sp. PCC6803

BACKGROUND

This study aimed at exploring the molecular physiological consequences of a major redirection of carbon flow in so-called cyanobacterial cell factories: quantitative whole-cell proteomics analyses were carried out on two (14)N-labelled Synechocystis mutant strains, relative to their (15)N-labelled wild-type counterpart. Each mutant strain overproduced one specific commodity product, i.e. ethanol or lactic acid, to such an extent that the majority of the incoming CO2 in the organism was directly converted into the product.

RESULTS

In total, 267 proteins have been identified with a significantly up- or down-regulated expression level. In the ethanol-producing mutant, which had the highest relative direct flux of carbon-to-product (>65%), significant up-regulation of several components involved in the initial stages of CO2 fixation for cellular metabolism was detected. Also a general decrease in abundance of the protein synthesizing machinery of the cells and a specific induction of an oxidative stress response were observed in this mutant. In the lactic acid overproducing mutant, that expresses part of the heterologous l-lactate dehydrogenase from a self-replicating plasmid, specific activation of two CRISPR associated proteins, encoded on the endogenous pSYSA plasmid, was observed. RT-qPCR was used to measure, of nine of the genes identified in the proteomics studies, also the adjustment of the corresponding mRNA level.

CONCLUSION

The most striking adjustments detected in the proteome of the engineered cells were dependent on the specific product formed, with, e.g. more stress caused by lactic acid- than by ethanol production. Up-regulation of the total capacity for CO2 fixation in the ethanol-producing strain was due to hierarchical- rather than metabolic regulation. Furthermore, plasmid-based expression of heterologous gene(s) may induce genetic instability. For selected, limited, number of genes a striking correlation between the respective mRNA- and the corresponding protein expression level was observed, suggesting that for the expression of these genes regulation takes place primarily at the level of gene transcription.

Oxygenic photosynthesis without galactolipids

The thylakoid membranes of oxygenic photosynthetic organisms are dominated by the galactolipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG). In cyanobacteria, MGDG is synthesized via monoglucosyldiacylglycerol (GlcDG). However, the putative epimerase involved in the conversion of GlcDG to MGDG has not been identified. Here we report the identification of the gene for the glucolipid epimerase (mgdE) by comparative genomic analysis. Knockout mutants of mgdE in Synechocystis sp. PCC 6803 lacked both MGDG and DGDG and accumulated GlcDG. The mutants did possess thylakoid membranes and showed normal maximal photosynthetic activity, albeit with reduced utilization of light energy. These results cast doubt on the long-standing belief that oxygenic photosynthesis is absolutely dependent on galactolipids.

Identification and stress-induced expression of three 3β-hydroxysteroid dehydrogenases from Erysimum crepidifolium Rchb. and their putative role in cardenolide biosynthesis

3β-Hydroxysteroid dehydrogenases (3βHSD) are supposed to be involved in cardenolide biosynthesis in plants. Erysimum crepidifolium Rchb., a member of the Brassicaceae accumulating cardenolides, is a close relative to Arabidopsis thaliana. Full length cDNAs encoding for three individual 3βHSDs (EcHSD1, EcHSD2, EcHSD3) were isolated from E. crepidifolium leaves. EcHSD1 and EcHSD2 encode proteins assembled from 257 amino acids whereas EcHSD3 encodes a protein assembled from 260 amino acids. All three proteins qualify as members of the short-chain dehydrogenases/reductases family of proteins (SDRs). EcHSD1 and EcHSD2 shared a high amino acid sequence identity of about 86% and 91% with putative 3βHSDs of A. thaliana (AT2G47140 and AT2G47130). EcHSD3 showed high homology to the A. thaliana SDRs AT2G47150 (74%) and AT2G47120 (81%). All three EcHSD genes were expressed in Escherichia coli and the recombinant enzymes were characterized biochemically. All three recombinant EcHSDs catalyzed the dehydrogenation of pregnenolone and the 3-reduction of 5α/β-pregnane-3,20-dione when NAD and NADH were used as cosubstrates, respectively. After exposure to different stress conditions, no increased transcription was seen for EcHSD1 whereas EcHSD2 was expressed four times higher under osmotic stress than under control conditions. EcHSD3 expression was 10 times and 6 times higher after osmotic stress and MeJA treatment, respectively, than in controls.

The plant short-chain dehydrogenase (SDR) superfamily: genome-wide inventory and diversification patterns

BACKGROUND

Short-chain dehydrogenases/reductases (SDRs) form one of the largest and oldest NAD(P)(H) dependent oxidoreductase families. Despite a conserved 'Rossmann-fold' structure, members of the SDR superfamily exhibit low sequence similarities, which constituted a bottleneck in terms of identification. Recent classification methods, relying on hidden-Markov models (HMMs), improved identification and enabled the construction of a nomenclature. However, functional annotations of plant SDRs remain scarce.

RESULTS

Wide-scale analyses were performed on ten plant genomes. The combination of hidden Markov model (HMM) based analyses and similarity searches led to the construction of an exhaustive inventory of plant SDR. With 68 to 315 members found in each analysed genome, the inventory confirmed the over-representation of SDRs in plants compared to animals, fungi and prokaryotes. The plant SDRs were first classified into three major types - 'classical', 'extended' and 'divergent' - but a minority (10% of the predicted SDRs) could not be classified into these general types ('unknown' or 'atypical' types). In a second step, we could categorize the vast majority of land plant SDRs into a set of 49 families. Out of these 49 families, 35 appeared early during evolution since they are commonly found through all the Green Lineage. Yet, some SDR families - tropinone reductase-like proteins (SDR65C), 'ABA2-like'-NAD dehydrogenase (SDR110C), 'salutaridine/menthone-reductase-like' proteins (SDR114C), 'dihydroflavonol 4-reductase'-like proteins (SDR108E) and 'isoflavone-reductase-like' (SDR460A) proteins - have undergone significant functional diversification within vascular plants since they diverged from Bryophytes. Interestingly, these diversified families are either involved in the secondary metabolism routes (terpenoids, alkaloids, phenolics) or participate in developmental processes (hormone biosynthesis or catabolism, flower development), in opposition to SDR families involved in primary metabolism which are poorly diversified.

CONCLUSION

The application of HMMs to plant genomes enabled us to identify 49 families that encompass all Angiosperms ('higher plants') SDRs, each family being sufficiently conserved to enable simpler analyses based only on overall sequence similarity. The multiplicity of SDRs in plant kingdom is mainly explained by the diversification of large families involved in different secondary metabolism pathways, suggesting that the chemical diversification that accompanied the emergence of vascular plants acted as a driving force for SDR evolution.