Labeling Studies Clarify the Committed Step in Bacterial Gibberellin Biosynthesis

Production of Gibberellins via a Non-Natural Pathway Using Steviol as a Substrate

Gibberellins (GAs) are plant hormones widely used in agriculture. At present, GAs are produced by fermentation of the fungus Fusarium fujikuroi. However, fungal growth is too slow, resulting in slow fungal fermentation and a low yield. Here, to develop an alternative production source of GAs, an artificial pathway was engineered in Escherichia coli. By selecting and combining enzymes derived from plants and bacteria, a novel 4-enzyme pathway was successfully constructed to produce GAs using steviol, a readily available and less valuable byproduct during enzymatic refining of rebaudioside A, as a feedstock. Whole-cell biotransformation with E. coli strain expressing the novel pathway produced 71.17 ± 2.00 mg/L GA1 and a trace amount of GA3 from steviol in 48 h. This report presents a significant advancement in the fast production of GAs and establishes a method for the metabolism of terpenoids to produce target products in microbial hosts.

Dual factors required for cytochrome-P450-mediated hydrocarbon ring contraction in bacterial gibberellin phytohormone biosynthesis

Cytochromes P450 (CYPs) are heme-thiolate monooxygenases that prototypically catalyze the insertion of oxygen into unactivated C-H bonds but are capable of mediating more complex reactions. One of the most remarked-upon alternative reactions occurs during biosynthesis of the gibberellin A (GA) phytohormones, involving hydrocarbon ring contraction with coupled aldehyde extrusion of ent-kaurenoic acid to form the first gibberellin intermediate. While the unusual nature of this reaction has long been noted, its mechanistic basis has remained opaque. Building on identification of the relevant CYP114 from bacterial GA biosynthesis, detailed structure-function studies are reported here, including development of in vitro assays as well as crystallographic analyses both in the absence and presence of substrate. These structures provided insight into enzymatic catalysis of this unusual reaction, as exemplified by identification of a key role for the "missing" acid from an otherwise highly conserved acid-alcohol pair of residues. Notably, the results demonstrate that ring contraction requires dual factors, both the use of a dedicated ferredoxin and absence of the otherwise conserved acidic residue, with exclusion of either limiting turnover to just the initiating and more straightforward hydroxylation. The results provide detailed insight into the enzymatic structure-function relationships underlying this fascinating reaction and support the use of a semipinacol mechanism for the unusual ring contraction reaction.

Metabolic Profiling for the Discovery of Structurally Diverse Gibberellins and Their Precursors from the Endophytic Fungus Fusarium sp. NJ-F5

Gibberellins (GAs) are well-known tetracyclic diterpenoid phytohormones since the 1950s. In this work, eight skeletally diverse GAs (1-8) including four new compounds (1-4), and three known ent-kaurene diterpenoids (9-11), were isolated from the endophytic fungus Fusarium sp. NJ-F5 by integrating mass spectrometry (MS)- and nuclear magnetic resonance (NMR)-based metabolic profiling. Their planar structures and stereochemistry were determined by extensive spectroscopic analyses including MS, NMR, as well as electronic circular dichroism and their calculations, together with single-crystal X-ray diffraction studies. As far as we know, this is a rare report of naturally occurring GAs and their detailed spectroscopic data including MS and NMR in recent decades. Compound 1, as a new member of GAs family, showed an obvious promoting effect on the seedling's growth ofArabidopsis thaliana.

The Current Status of Research on Gibberellin Biosynthesis

Gibberellins are produced by all vascular plants and several fungal and bacterial species that associate with plants as pathogens or symbionts. In the 60 years since the first experiments on the biosynthesis of gibberellic acid in the fungus Fusarium fujikuroi, research on gibberellin biosynthesis has advanced to provide detailed information on the pathways, biosynthetic enzymes and their genes in all three kingdoms, in which the production of the hormones evolved independently. Gibberellins function as hormones in plants, affecting growth and differentiation in organs in which their concentration is very tightly regulated. Current research in plants is focused particularly on the regulation of gibberellin biosynthesis and inactivation by developmental and environmental cues, and there is now considerable information on the molecular mechanisms involved in these processes. There have also been recent advances in understanding gibberellin transport and distribution and their relevance to plant development. This review describes our current understanding of gibberellin metabolism and its regulation, highlighting the more recent advances in this field.

Bacterial Diterpene Biosynthesis

This Minireview summarises recent developments in the biosynthesis of diterpenes by diterpene synthases in bacteria. It is structured by the class of enzyme involved in the first committed step towards diterpenes, starting with type I diterpene synthases, followed by type II enzymes and the more recently discovered UbiA-related diterpene synthases. A special emphasis lies on the reaction mechanisms of diterpene synthases that convert simple linear precursors through cationic cascades into structurally complex, usually polycyclic carbon skeletons with multiple stereogenic centres. A further main focus of this Minireview is a discussion of how these mechanisms can be unravelled. Downstream modifications to bioactive molecules are also covered.

Cryptic and Stereospecific Hydroxylation, Oxidation, and Reduction in Platensimycin and Platencin Biosynthesis

Platensimycin (PTM) and platencin (PTN) are highly functionalized bacterial diterpenoids of ent-kauranol and ent-atiserene biosynthetic origin. C7 oxidation in the B-ring plays a key biosynthetic role in generating structural complexity known for ent-kaurane and ent-atisane derived diterpenoids. While all three oxidation patterns, α-hydroxyl, β-hydroxyl, and ketone, at C7 are seen in both the ent-kaurane and ent-atisane derived diterpenoids, their biosynthetic origins remain largely unknown. We previously established that PTM and PTN are produced by a single biosynthetic machinery, featuring cryptic C7 oxidations at the B-rings that transform the ent-kauranol and ent-atiserene derived precursors into the characteristic PTM and PTN scaffolds. Here, we report a three-enzyme cascade affording C7 α-hydroxylation in PTM and PTN biosynthesis. Combining in vitro and in vivo studies, we show that PtmO3 and PtmO6 are two functionally redundant α-ketoglutarate-dependent dioxygenases that generate a cryptic C7 β-hydroxyl on each of the ent-kauranol and ent-atiserene scaffolds, and PtmO8 and PtmO1, a pair of NAD⁺/NADPH-dependent dehydrogenases, subsequently work in concert to invert the C7 β-hydroxyl to α-hydroxyl via a C7 ketone intermediate. PtmO3 and PtmO6 represent the first dedicated C7 β-hydroxylases characterized to date and, together with PtmO8 and PtmO1, provide an account for the biosynthetic origins of all three C7 oxidation patterns that may shed light on other B-ring modifications in bacterial, plant, and fungal diterpenoid biosynthesis. Given their unprecedented activities in C7 oxidations, PtmO3, PtmO6, PtmO8, and PtmO1 enrich the growing toolbox of novel enzymes that could be exploited as biocatalysts to rapidly access complex diterpenoid natural products.

Retracted Article: Synthesis of deuterated isopentyl pyrophosphates for chemo-enzymatic labelling methods: GC-EI-MS based 1,2-hydride shift in epicedrol biosynthesis

A sesquiterpene epicedrol cyclase mechanism was elucidated based on the gas chromatography coupled to electron impact mass spectrometry fragmentation data of deuterated (²H) epicedrol analogues. The chemo-enzymatic method was applied for the specific synthesis of 8-position labelled farnesyl pyrophosphate and epicedrol. EI-MS fragmentation ions compared with non-labelled and isotopic mass shift fragments suggest that the ²H of C6 migrates to the C7 position during the cyclization mechanism.

The cyclisation mechanism of epicedrol cyclase elucidated based on GC-EI-MS fragmentation of specific deuterated (²H) epicedrol analogues. The chemo-enzymatic method was applied for the synthesis 8-position-²H-farnesyl pyrophosphate synthesis.

A Third Class: Functional Gibberellin Biosynthetic Operon in Beta-Proteobacteria

The ability of plant-associated microbes to produce gibberellin A (GA) phytohormones was first described for the fungal rice pathogen Gibberella fujikuroi in the 1930s. Recently the capacity to produce GAs was shown for several bacteria, including symbiotic alpha-proteobacteria (α-rhizobia) and gamma-proteobacteria phytopathogens. All necessary enzymes for GA production are encoded by a conserved operon, which appears to have undergone horizontal transfer between and within these two phylogenetic classes of bacteria. Here the operon was shown to be present and functional in a third class, the beta-proteobacteria, where it is found in several symbionts (β-rhizobia). Conservation of function was examined by biochemical characterization of the enzymes encoded by the operon from Paraburkholderia mimosarum LMG 23256^T. Despite the in-frame gene fusion between the short-chain alcohol dehydrogenase/reductase and ferredoxin, the encoded enzymes exhibited the expected activity. Intriguingly, together these can only produce GA₉, the immediate precursor to the bioactive GA₄, as the cytochrome P450 (CYP115) that catalyzes the final hydroxylation reaction is missing, similar to most α-rhizobia. However, phylogenetic analysis indicates that the operon from β-rhizobia is more closely related to examples from gamma-proteobacteria, which almost invariably have CYP115 and, hence, can produce bioactive GA₄. This indicates not only that β-rhizobia acquired the operon by horizontal gene transfer from gamma-proteobacteria, rather than α-rhizobia, but also that they independently lost CYP115 in parallel to the α-rhizobia, further hinting at the possibility of detrimental effects for the production of bioactive GA₄ by these symbionts.

Investigating the Phylogenetic Range of Gibberellin Biosynthesis in Bacteria

Certain plant-associated microbes can produce gibberellin (GA) phytohormones, as first described for the rice fungal pathogen Gibberella fujikuroi and, more recently, for bacteria, including several rhizobia and the rice bacterial pathogen Xanthomonas oryzae pv. oryzicola. The relevant enzymes are encoded by a biosynthetic operon that exhibits both a greater phylogenetic range and scattered distribution among plant-associated bacteria. Here, the phylogenetic distribution of this operon was investigated. To demonstrate conserved functionality, the enzymes encoded by the disparate operon from X. translucens pv. translucens, along with those from the most divergent example, found in Erwinia tracheiphila, were biochemically characterized. In both of these phytopathogens, the operon leads to production of the bioactive GA4. Based on these results, it seems that this operon is widely dedicated to GA biosynthesis. However, there is intriguing variation in the exact product. In particular, although all plant pathogens seem to produce bioactive GA4, rhizobia generally only produce the penultimate hormonal precursor GA9. This is suggested to reflect their distinct interactions with plants, because production of GA4 counteracts the jasmonic-acid-mediated defense response, reflecting the importance of wounds as the entry point for these phytopathogens, whereas such suppression presumably is detrimental in the rhizobial symbiotic relationship.

Hot off the press

A personal selection of 32 recent papers is presented covering various aspects of current developments in bioorganic chemistry and novel natural products such as hitorin A from Chloranthus japonicus.

Diterpenoids of terrestrial origin

This review covers the isolation and chemistry of diterpenoids from terrestrial as opposed to marine sources and includes labdanes, clerodanes, abietanes, pimaranes, kauranes, cembranes and their cyclization products. The literature from January to December, 2016 is reviewed.