Effect of mutations in the transmethylase and dehydrogenase/chelatase domains of sirohaem synthase (CysG) on sirohaem and cobalamin biosynthesis

Complete genome sequence and transcriptome response to vitamin C supplementation of Novacetimonas hansenii SI1 - producer of highly-stretchable cellulose

Novacetimonas hansenii SI1, previously known as Komagataeibacter hansenii, produces bacterial nanocellulose (BNC) with unique ability to stretch. The addition of vitamin C in the culture medium increases the porosity of the membranes and their stretchability making them highly moldable. To better understand the genetic background of this strain, we obtained its complete genome sequence using a hybrid sequencing and assembly strategy. We described the functional regions in the genome which are important for the synthesis of BNC and acetan-like II polymer. We next investigated the effect of 1% vitamin C supplementation on the global gene expression profile using RNA sequencing. Our transcriptomic readouts imply that vitamin C functions mainly as a reducing agent. We found that the changes in cellular redox status are balanced by strong repression of the sulfur assimilation pathway. Moreover, in the reduced conditions, glucose oxidation is decreased and alternative pathways for energy generation, such as acetate accumulation, are activated. The presence of vitamin C negatively influences acetan-like II polymer biosynthesis, which may explain the lowered yield and changed mechanical properties of BNC. The results of this study enrich the functional characteristics of the genomes of the efficient producers of the N. hansenii species. Improved understanding of the adaptation to the presence of vitamin C at the molecular level has important guiding significance for influencing the biosynthesis of BNC and its morphology.

His-Ala-Phe-Lys peptide from Burkholderia arboris possesses antifungal activity

Burkholderia arboris, which belongs to the Burkholderia cepacia complex, has been shown to possess antifungal activity against several plant fungal pathogens; however, the antifungal compounds are yet to be identified. Here, we identified the antifungal compounds produced by B. arboris using genetic and metabolomic approaches. We generated a Tn5 transposon mutation library of 3,000 B. arboris mutants and isolated three mutants with reduced antifungal activity against the plant fungal pathogen Fusarium oxysporum. Among the mutants, the M464 mutant exhibited the weakest antifungal activity. In the M464 genome, the transposon was inserted into the cobA gene, encoding uroporphyrin-III methyltransferase. Deletion of the cobA gene also resulted in reduced antifungal activity, indicating that the cobA gene contributed to the antifungal activity of B. arboris. Furthermore, a comparison of the differential metabolites between wild type B. arboris and the ∆cobA mutant showed a significantly decreased level of tetrapeptide His-Ala-Phe-Lys (Hafk) in the ∆cobA mutant. Therefore, a Hafk peptide with D-amino acid residues was synthesized and its antifungal activity was evaluated. Notably, the Hafk peptide displayed significant antifungal activity against F. oxysporum and Botrytis cinerea, two plant pathogens that cause destructive fungal diseases. Overall, a novel antifungal compound (Hafk) that can be used for the biocontrol of fungal diseases in plants was identified in B. arboris.

VB12Path for Accurate Metagenomic Profiling of Microbially Driven Cobalamin Synthesis Pathways

Cobalamin (vitamin B12; VB₁₂) is an indispensable nutrient for all living entities in the Earth’s biosphere and plays a vital role in both natural and host environments. Currently in the metagenomic era, gene families of interest are extracted and analyzed based on functional profiles by searching shotgun metagenomes against public databases. However, critical issues exist in applying public databases for specific processes such as VB₁₂ biosynthesis pathways. We developed a curated functional gene database termed VB₁₂Path for accurate metagenomic profiling of VB₁₂ biosynthesis gene families of microbial communities in complex environments. VB₁₂Path contains a total of 60 VB₁₂ synthesis gene families, 287,731 sequences, and 21,154 homology groups, and it aims to provide accurate functional and taxonomic profiles of VB₁₂ synthesis pathways for shotgun metagenomes and minimize false-positive assignments. VB₁₂Path was applied to characterize cobalamin biosynthesis gene families in human intestines and marine environments. The results demonstrated that ocean and human intestine had dramatically different VB₁₂ synthesis processes and that gene families belonging to salvage and remodeling pathway dominated human intestine but were lowest in the ocean ecosystem. VB₁₂Path is expected to be a useful tool to study cobalamin biosynthesis processes via shotgun metagenome sequencing in both environmental and human microbiome research.

IMPORTANCE Vitamin B12 (VB₁₂) is an indispensable nutrient for all living entities in the world but can only be synthesized by a small subset of prokaryotes. Therefore, this small subset of prokaryotes controls ecosystem stability and host health to some extent. However, critical accuracy and comprehensiveness issues exist in applying public databases to profile VB₁₂ synthetic gene families and taxonomic groups in complex metagenomes. In this study, we developed a curated functional gene database termed VB₁₂Path for accurate metagenomic profiling of VB₁₂ communities in complex environments. VB₁₂Path is expected to serve as a valuable tool to uncover the hidden microbial communities producing this precious nutrient on Earth.

Siroheme synthase orients substrates for dehydrogenase and chelatase activities in a common active site

Siroheme is the central cofactor in a conserved class of sulfite and nitrite reductases that catalyze the six-electron reduction of sulfite to sulfide and nitrite to ammonia. In Salmonella enterica serovar Typhimurium, siroheme is produced by a trifunctional enzyme, siroheme synthase (CysG). A bifunctional active site that is distinct from its methyltransferase activity catalyzes the final two steps, NAD⁺-dependent dehydrogenation and iron chelation. How this active site performs such different chemistries is unknown. Here, we report the structures of CysG bound to precorrin-2, the initial substrate; sirohydrochlorin, the dehydrogenation product/chelation substrate; and a cobalt-sirohydrochlorin product. We identified binding poses for all three tetrapyrroles and tested the roles of specific amino acids in both activities to give insights into how a bifunctional active site catalyzes two different chemistries and acts as an iron-specific chelatase in the final step of siroheme synthesis.

Siroheme is an essential bacterial iron tetrapyrrole used by siroheme-dependent sulfite and nitrite reductases. Here the authors shed light on the catalytic mechanisms of siroheme synthase through the structures of the bifunctional dehydrogenase/chelatase CysG module bound to its substrate, precorrin-2, the product/substrate sirohydrochlorin, and cobalt-sirohydrochlorin.

Transcriptome analysis reveals the pigmentation-related genes in two shell color strains of the Manila clam Ruditapes philippinarum

The Manila clam, Ruditapes philippinarum, is an ecologically and economically important marine bivalve species. In this study, we conducted transcriptomic sequencing of two different shell color strains (O and Z) before color appearance (uncolored juvenile clam) and pigmented shell color (colored juvenile clam) and investigated the analysis of the differential expression patterns of specific genes associated with pigmentation by RNA-seq and time course qPCR analysis. The transcription level of 16 differentially expressed genes (DEGs) related with shell color was analyzed by qRT-PCR to validate the performance of RNA-seq from Illumina sequence data where most of them were up-regulated. Two genes were down-regulated after the occurrence of zebra clam stripes compared with uncolored zebra clam. The trend of gene expression obtained by qPCR was basically consistent with that of RNA-seq. The synthesis of melanin in bivalves plays potential roles in the pigmentation of the shell and is closely related to the formation of the surface pattern. The porphyrin metabolism combined with tyrosinase and melanogenesis signaling pathway is a novel finding in shell color determination of R. philippinarum. This study sheds light on the pigmentation and coloration mechanism of the Manila clam.

Biosynthesis and Use of Cobalamin (B12)

This review summarizes research performed over the last 23 years on the genetics, enzyme structures and functions, and regulation of the expression of the genes encoding functions involved in adenosylcobalamin (AdoCbl, or coenzyme B12) biosynthesis. It also discusses the role of coenzyme B12 in the physiology of Salmonella enterica serovar Typhimurium LT2 and Escherichia coli. John Roth's seminal contributions to the field of coenzyme B12 biosynthesis research brought the power of classical and molecular genetic, biochemical, and structural approaches to bear on the extremely challenging problem of dissecting the steps of what has turned out to be one of the most complex biosynthetic pathways known. In E. coli and serovar Typhimurium, uro'gen III represents the first branch point in the pathway, where the routes for cobalamin and siroheme synthesis diverge from that for heme synthesis. The cobalamin biosynthetic pathway in P. denitrificans was the first to be elucidated, but it was soon realized that there are at least two routes for cobalamin biosynthesis, representing aerobic and anaerobic variations. The expression of the AdoCbl biosynthetic operon is complex and is modulated at different levels. At the transcriptional level, a sensor response regulator protein activates the transcription of the operon in response to 1,2-Pdl in the environment. Serovar Typhimurium and E. coli use ethanolamine as a source of carbon, nitrogen, and energy. In addition, and unlike E. coli, serovar Typhimurium can also grow on 1,2-Pdl as the sole source of carbon and energy.

Biosynthesis of Cysteine

The synthesis of L-cysteine from inorganic sulfur is the predominant mechanism by which reduced sulfur is incorporated into organic compounds. L-cysteineis used for protein and glutathione synthesis and serves as the primary source of reduced sulfur in L-methionine, lipoic acid, thiamin, coenzyme A (CoA), molybdopterin, and other organic molecules. Sulfate and thiosulfate uptake in E. coli and serovar Typhimurium are achieved through a single periplasmic transport system that utilizes two different but similar periplasmic binding proteins. Kinetic studies indicate that selenate and selenite share a single transporter with sulfate, but molybdate also has a separate transport system. During aerobic growth, the reduction of sulfite to sulfide is catalyzed by NADPH-sulfite reductase (SiR), and serovar Typhimurium mutants lacking this enzyme accumulate sulfite from sulfate, implying that sulfite is a normal intermediate in assimilatory sulfate reduction. L-Cysteine biosynthesis in serovar Typhimurium and E. coli ceases almost entirely when cells are grown on L-cysteine or L-cystine, owing to a combination of end product inhibition of serine transacetylase by L-cysteine and a gene regulatory system known as the cysteine regulon, wherein genes for sulfate assimilation and alkanesulfonate utilization are expressed only when sulfur is limiting. In vitro studies with the cysJIH, cysK, and cysP promoters have confirmed that they are inefficient at forming transcription initiation complexes without CysB and N-acetyl-L-serine. Activation of the tauA and ssuE promoters requires Cbl. It has been proposed that the three serovar Typhimurium anaerobic reductases for sulfite, thiosulfate, and tetrathionate may function primarily in anaerobic respiration.

Recent advances in microbial production of δ-aminolevulinic acid and vitamin B12

δ-aminolevulinate (ALA) is an important intermediate involved in tetrapyrrole synthesis (precursor for vitamin B12, chlorophyll and heme) in vivo. It has been widely applied in agriculture and medicine. On account of many disadvantages of its chemical synthesis, microbial production of ALA has been received much attention as an alternative because of less expensive raw materials, low pollution, and high productivity. Vitamin B12, one of ALA derivatives, which plays a vital role in prevention of anaemia has also attracted intensive works. In this review, recent advances on the production of ALA and vitamin B12 with novel approaches such as whole-cell enzyme-transformation and metabolic engineering are described. Furthermore, the direction for future research and perspective are also summarized.

Crystal structure of the heme d1 biosynthesis enzyme NirE in complex with its substrate reveals new insights into the catalytic mechanism of S-adenosyl-L-methionine-dependent uroporphyrinogen III methyltransferases

During the biosynthesis of heme d(1), the essential cofactor of cytochrome cd(1) nitrite reductase, the NirE protein catalyzes the methylation of uroporphyrinogen III to precorrin-2 using S-adenosyl-L-methionine (SAM) as the methyl group donor. The crystal structure of Pseudomonas aeruginosa NirE in complex with its substrate uroporphyrinogen III and the reaction by-product S-adenosyl-L-homocysteine (SAH) was solved to 2.0 Å resolution. This represents the first enzyme-substrate complex structure for a SAM-dependent uroporphyrinogen III methyltransferase. The large substrate binds on top of the SAH in a "puckered" conformation in which the two pyrrole rings facing each other point into the same direction either upward or downward. Three arginine residues, a histidine, and a methionine are involved in the coordination of uroporphyrinogen III. Through site-directed mutagenesis of the nirE gene and biochemical characterization of the corresponding NirE variants the amino acid residues Arg-111, Glu-114, and Arg-149 were identified to be involved in NirE catalysis. Based on our structural and biochemical findings, we propose a potential catalytic mechanism for NirE in which the methyl transfer reaction is initiated by an arginine catalyzed proton abstraction from the C-20 position of the substrate.

Cloning and heterologous expression of Lactobacillus reuteri uroporphyrinogen III synthase/methyltransferase gene (cobA/hemD): preliminary characterization

PURPOSE OF WORK

To clone, express and characterize uroporphyrinogen III synthase/methyltransferase gene (cobA/hemD) from Lactobacillus reuteri. Some strains of Lb. reuteri produce cobalamin (vitamin B(12)). Cobalamin biosynthesis relies on the sequential action of more than 25 enzymes in a complex metabolic pathway. We have cloned, expressed and characterized the gene in Lb. reuteri that codes for the S-adenosy L: -methionine uroprophyrinogen III methyltransferase/synthase (CobA/HemD), a key bifunctional enzyme in the biosynthesis of cobalamin and other tetrapyrrols.

Insight into the haem d1 biosynthesis pathway in heliobacteria through bioinformatics analysis

Haem d(1) is a unique tetrapyrrole molecule that serves as a prosthetic group of cytochrome cd(1), which reduces nitrite to nitric oxide during the process of denitrification. Very little information is available regarding the biosynthesis of haem d(1). The extreme difficulty in studying the haem d(1) biosynthetic pathway can be partly attributed to the lack of a theoretical basis for experimental investigation. We report here a gene cluster encoding enzymes involved in the biosynthesis of haem d(1) in two heliobacterial species, Heliobacillus mobilis and Heliophilum fasciatum. The gene organization of the cluster is conserved between the two species, and contains a complete set of genes that lead to the biosynthesis of uroporphyrinogen III and genes thought to be involved in the late steps of haem d(1) biosynthesis. Detailed bioinformatics analysis of some of the proteins encoded in the gene cluster revealed important clues to the precise biochemical roles of the proteins in the biosynthesis of haem d(1), as well as the membrane transport and insertion of haem d(1) into an apocytochrome during the maturation of cytochrome cd(1).

Sulfite reduction in mycobacteria

Mycobacterium tuberculosis places an enormous burden on the welfare of humanity. Its ability to grow and its pathogenicity are linked to sulfur metabolism, which is considered a fertile area for the development of antibiotics, particularly because many of the sulfur acquisition steps in the bacterium are not found in the host. Sulfite reduction is one such mycobacterium-specific step and is the central focus of this paper. Sulfite reduction in Mycobacterium smegmatis was investigated using a combination of deletion mutagenesis, metabolite screening, complementation, and enzymology. The initial rate parameters for the purified sulfite reductase from M. tuberculosis were determined under strict anaerobic conditions [k(cat) = 1.0 (+/-0.1) electron consumed per second, and K(m(SO(3)(-2))) = 27 (+/-1) microM], and the enzyme exhibits no detectible turnover of nitrite, which need not be the case in the sulfite/nitrite reductase family. Deletion of sulfite reductase (sirA, originally misannotated nirA) reveals that it is essential for growth on sulfate or sulfite as the sole sulfur source and, further, that the nitrite-reducing activities of the cell are incapable of reducing sulfite at a rate sufficient to allow growth. Like their nitrite reductase counterparts, sulfite reductases require a siroheme cofactor for catalysis. Rv2393 (renamed che1) resides in the sulfur reduction operon and is shown for the first time to encode a ferrochelatase, a catalyst that inserts Fe(2+) into siroheme. Deletion of che1 causes cells to grow slowly on metabolites that require sulfite reductase activity. This slow-growth phenotype was ameliorated by optimizing growth conditions for nitrite assimilation, suggesting that nitrogen and sulfur assimilation overlap at the point of ferrochelatase synthesis and delivery.

Biosynthesis of Hemes

This review is concerned specifically with the structures and biosynthesis of hemes in E. coli and serovar Typhimurium. However, inasmuch as all tetrapyrroles share a common biosynthetic pathway, much of the material covered here is applicable to tetrapyrrole biosynthesis in other organisms. Conversely, much of the available information about tetrapyrrole biosynthesis has been gained from studies of other organisms, such as plants, algae, cyanobacteria, and anoxygenic phototrophs, which synthesize large quantities of these compounds. This information is applicable to E. coli and serovar Typhimurium. Hemes play important roles as enzyme prosthetic groups in mineral nutrition, redox metabolism, and gas-and redox-modulated signal transduction. The biosynthetic steps from the earliest universal precursor, 5-aminolevulinic acid (ALA), to protoporphyrin IX-based hemes constitute the major, common portion of the pathway, and other steps leading to specific groups of products can be considered branches off the main axis. Porphobilinogen (PBG) synthase (PBGS; also known as ALA dehydratase) catalyzes the asymmetric condensation of two ALA molecules to form PBG, with the release of two molecules of H2O. Protoporphyrinogen IX oxidase (PPX) catalyzes the removal of six electrons from the tetrapyrrole macrocycle to form protoporphyrin IX in the last biosynthetic step that is common to hemes and chlorophylls. Several lines of evidence converge to support a regulatory model in which the cellular level of available or free protoheme controls the rate of heme synthesis at the level of the first step unique to heme synthesis, the formation of GSA by the action of GTR.

Structure/function studies on a S-adenosyl-L-methionine-dependent uroporphyrinogen III C methyltransferase (SUMT), a key regulatory enzyme of tetrapyrrole biosynthesis

The crystallographic structure of the Pseudomonas denitrificans S-adenosyl-L-methionine-dependent uroporphyrinogen III methyltransferase (SUMT), which is encoded by the cobA gene, has been solved by molecular replacement to 2.7A resolution. SUMT is a branchpoint enzyme that plays a key role in the biosynthesis of modified tetrapyrroles by controlling flux to compounds such as vitamin B(12) and sirohaem, and catalysing the transformation of uroporphyrinogen III into precorrin-2. The overall topology of the enzyme is similar to that of the SUMT module of sirohaem synthase (CysG) and the cobalt-precorrin-4 methyltransferase CbiF and, as with the latter structures, SUMT has the product S-adenosyl-L-homocysteine bound in the crystal. The roles of a number of residues within the SUMT structure are discussed with respect to their conservation either across the broader family of cobalamin biosynthetic methyltransferases or within the sub-group of SUMT members. The D47N, L49A, F106A, T130A, Y183A and M184A variants of SUMT were generated by mutagenesis of the cobA gene, and tested for SAM binding and enzymatic activity. Of these variants, only D47N and L49A bound the co-substrate S-adenosyl-L-methionine. Consequently, all the mutants were severely restricted in their capacity to synthesise precorrin-2, although both the D47N and L49A variants produced significant quantities of precorrin-1, the monomethylated derivative of uroporphyrinogen III. The activity of these variants is interpreted with respect to the structure of the enzyme.

CysG structure reveals tetrapyrrole-binding features and novel regulation of siroheme biosynthesis

Sulfur metabolism depends on the iron-containing porphinoid siroheme. In Salmonella enterica, the S-adenosyl-L-methionine (SAM)-dependent bismethyltransferase, dehydrogenase and ferrochelatase, CysG, synthesizes siroheme from uroporphyrinogen III (uro'gen III). The reactions mediated by CysG encompass two branchpoint intermediates in tetrapyrrole biosynthesis, diverting flux first from protoporphyrin IX biosynthesis and then from cobalamin (vitamin B(12)) biosynthesis. We determined the first structure of this multifunctional siroheme synthase by X-ray crystallography. CysG is a homodimeric gene fusion product containing two structurally independent modules: a bismethyltransferase and a dual-function dehydrogenase-chelatase. The methyltransferase active site is a deep groove with a hydrophobic patch surrounded by hydrogen bond donors. This asymmetric arrangement of amino acids may be important in directing substrate binding. Notably, our structure shows that CysG is a phosphoprotein. From mutational analysis of the post-translationally modified serine, we suggest a conserved role for phosphorylation in inhibiting dehydrogenase activity and modulating metabolic flux between siroheme and cobalamin pathways.

The structure of Saccharomyces cerevisiae Met8p, a bifunctional dehydrogenase and ferrochelatase

Sirohaem is a tetrapyrrole-derived prosthetic group that is required for the essential assimilation of sulfur and nitrogen into all living systems as part of the sulfite and nitrite reductase systems. The final two steps in the biosynthesis of sirohaem involve a beta-NAD(+)-dependent dehydrogenation of precorrin-2 to generate sirohydrochlorin followed by ferrochelation to yield sirohaem. In Saccharomyces cerevisiae, Met8p is a bifunctional enzyme that carries out both of these reactions. Here, we report the 2.2 A resolution crystal structure of Met8p, which adopts a novel fold that bears no resemblance to the previously determined structures of cobalt- or ferro-chelatases. Analysis of mutant proteins suggests that both catalytic activities share a single active site, and that Asp141 plays an essential role in both dehydrogenase and chelatase processes.

Identification of amino acid residues of nitrite reductase from Anabaena sp. PCC 7120 involved in ferredoxin binding

The nitrite reductase gene (nirA) from the filamentous, heterocyst-forming cyanobacterium Anabaena sp. PCC 7120 (A. PCC 7120) was expressed in Escherichia coli using the pET-system. Co-expression of the cysG gene encoding siroheme synthase of Salmonella typhimurium increased the amount of soluble, active nitrite reductase four fold. Nitrite reductase was purified to homogeneity. In order to identify amino acid residues involved in ferredoxin (PetF)-nitrite reductase electron transfer in A. PCC 7120, we performed a sequence comparison between ferredoxin-dependent nitrite reductases from various species. The alignment revealed a number of conserved residues possibly involved in ferredoxin nitrite reductase interaction. The position of these residues relative to the [4Fe4S]-cluster as the primary electron acceptor was tentatively localized in a three dimensional structure of the sulfite reductase from E. coli, which is closest related to nitrite reductase among the proteins with known tertiary structure. The exchange of certain positively charged amino acid residues of the nitrite reductase with uncharged residues revealed the influence of these residues on the interaction of nitrite reductase with reduced ferredoxin. We identified at least two separate regions of nitrite reductase that contribute to the binding of ferredoxin.

A Clostridium perfringens hem gene cluster contains a cysG(B) homologue that is involved in cobalamin biosynthesis

The hem gene cluster, which consists of hemA, cysG(B), hemC, hemD, hemB, and hemL genes, and encodes enzymes involved in the biosynthetic pathway from glutamyl-tRNA to uroporphyrinogen III, has been identified by the cloning and sequencing of two overlapping DNA fragments from Clostridium perfringens NCTC8237. The deduced amino acid sequence of the N-terminal region of C. perfringens HemD is homologous to those reported for the C-terminal region of Salmonella typhimurium CysG and Clostridium josui HemD. C. perfringens CysG(B) is a predicted 220-residue protein which shows homology to the N-terminal region of S. typhimurium CysG. Disruption of the cysG(B) gene in C. perfringens strain 13 by homologous recombination reduced cobalamin (vitamin B12) levels by a factor of 200. When grown in vitamin B12-deficient medium, the mutant strain showed a four-fold increase in its doubling time compared with that of the wild-type strain, and this effect was counteracted by supplementing the medium with vitamin B12. These results suggest that C. perfringens CysG(B) is involved in the chelation of cobalt to precorrin II as suggested for the CysG(B) domain of S. typhimurium CysG, enabling the synthesis of cobalamin.

The role of Saccharomyces cerevisiae Met1p and Met8p in sirohaem and cobalamin biosynthesis

MET1 and MET8 mutants of Saccharomyces cerevisiae can be complemented by Salmonella typhimurium cysG, indicating that the genes are involved in the transformation of uroporphyrinogen III into sirohaem. In the present study, we have demonstrated complementation of defined cysG mutants of Sal. typhimurium and Escherichia coli, with either MET1 or MET8 cloned in tandem with Pseudomonas denitrificans cobA. The conclusion drawn from these experiments is that MET1 encodes the S-adenosyl-l-methionine uroporphyrinogen III transmethylase activity, and MET8 encodes the dehydrogenase and chelatase activities (all three functions are encoded by Sal. typhimurium and E. coli cysG). MET8 was further cloned into pET14b to allow expression of the protein with an N-terminal His-tag. After purification, the functions of the His-tagged Met8p were studied in vitro by assay with precorrin-2 in the presence of NAD+ and Co2+. The results demonstrated that Met8p acts as a dehydrogenase and chelatase in the biosynthesis of sirohaem. Moreover, despite the fact that S. cerevisiae does not make cobalamins de novo, we have shown also that MET8 is able to complement cobalamin cobaltochelatase mutants and have revealed a subtle difference in the early stages of the anaerobic cobalamin biosynthetic pathways between Sal. typhimurium and Bacillus megaterium.

The X-ray structure of a cobalamin biosynthetic enzyme, cobalt-precorrin-4 methyltransferase

Biosynthesis of the corrin ring of vitamin B12 requires the action of six S-adenosyl-L-methionine (AdoMet) dependent transmethylases, closely related in sequence. The first X-ray structure of one of these, cobalt-precorrin-4 transmethylase, CbiF, from Bacillus megaterium has been determined to a resolution of 2.4 A. CbiF contains two alphabeta domains forming a trough in which S-adenosyl-L-homocysteine (AdoHcy) binds. The location of AdoHcy and a number of conserved residues, helps define the precorrin binding site. A second crystal form determined at 3.1 A resolution highlights the flexibility of two loops around this site. CbiF employs a unique mode of AdoHcy binding and represents a new class of transmethylase.