Biotin Synthase Mechanism: Mutagenesis of the YNHNLD Conserved Motif†,‡

Biotin synthase, a member of the "radical SAM" family, catalyzes the final step of the biotin biosynthetic pathway, namely, the insertion of a sulfur atom into dethiobiotin (DTB). The active form of the enzyme contains two iron-sulfur clusters, a [4Fe-4S](2+) cluster liganded by Cys-53, Cys-57, and Cys-60 and the S-adenosylmethionine (AdoMet or SAM) cosubstrate and a [2Fe-2S](2+) cluster liganded by Cys-97, Cys-128, Cys-188, and Arg-260. Single-point mutation of each of these six conserved cysteines produced inactive variants. In this work, mutants of other highly conserved residues from the Y(150)NHNLD motif are described. They have properties similar to those of the wild-type enzyme with respect to their cluster content and characteristics. For all of them, the as-isolated form, which contains an air-stable [2Fe-2S](2+) center, can additionally accommodate an air-sensitive [4Fe-4S](2+) center which is generated by incubation under anaerobic conditions with Fe(2+) and S(2-). Their spectroscopic properties are similar to those of the wild type. However, they are inactive, except the mutant H152A that exhibits a weak activity. We show that the mutants, inactive in producing biotin, are also unable to cleave AdoMet and to produce the deoxyadenosyl radical (AdoCH(2)(*)). In the case of H152A, a value of 5.5 +/- 0.4 is found for the 5'-deoxyadenosine (AdoCH(3)):biotin ratio, much higher than the value of 2.8 +/- 0.3 usually observed with the wild type. This reveals a greater contribution of the abortive process in which the AdoCH(2)(*) radical is quenched by hydrogen atoms from the protein or from some components of the system. Thus, in this case, the coupling between the production of AdoCH(2)(*) and its reaction with the hydrogen at C-6 and C-9 of DTB is less efficient than that in the wild type, probably because of geometry's perturbation within the active site.

Crystallization and preliminary structure analysis of CobE, an essential protein of cobalamin (vitamin B12) biosynthesis

CobE, a protein implicated in vitamin B12 biosynthesis, from Pseudomonas aeruginosa has been overexpressed in Escherichia coli, purified and crystallized using hanging-drop vapour diffusion. The crystals belong to the primitive orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 31.86, b = 41.07, c = 87.41 A. The diffraction extends to a resolution of 1.9 A. There is one molecule per asymmetric unit and the estimated solvent content is 35%. SeMet-labelled CobE has been prepared and crystallizes under the same conditions as the native protein with diffraction to 1.7 A. The anomalous measurements will be used for phasing.

Identification and Characterization of a Novel Vitamin B12 (Cobalamin) Biosynthetic Enzyme (CobZ) from Rhodobacter capsulatus, Containing Flavin, Heme, and Fe-S Cofactors

One of the most intriguing steps during cobalamin (vitamin B12) biosynthesis is the ring contraction process that leads to the extrusion of one of the integral macrocyclic carbon atoms from the tetrapyrrole-derived framework. The aerobic cobalamin pathway requires the action of a monooxygenase called CobG (precorrin-3B synthase), which generates a hydroxylactone intermediate that is subsequently ring-contracted by CobJ. However, in the photosynthetic bacterium Rhodobacter capsulatus, which harbors an aerobic-like pathway, there is no cobG in the main cobalamin biosynthetic operon although it does contain an additional uncharacterized gene called orf663. To demonstrate the involvement of Orf663 in cobalamin synthesis, the first dedicated 10 genes of the B12 pathway (including orf663), encoding enzymes for the transformation of uroporphyrinogen III into hydrogenobyrinic acid (HBA), were sequentially cloned into a plasmid to generate an artificial operon, which, when transformed into Escherichia coli, endowed the host with the ability to make HBA. Deletion of orf663 from this operon prevented HBA synthesis, demonstrating that it was essential for corrin construction. HBA synthesis was restored to this recombinant strain either by returning orf663 or by substituting it with cobG. Recombinant overproduction of Orf663, now renamed CobZ, allowed the characterization of a novel cofactor-rich protein, housing two Fe-S centers, a flavin, and a heme group, which like B12 itself is a modified tetrapyrrole. A mechanism for Orf663 (CobZ) in cobalamin biosynthesis is proposed.

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.

Characterization of the cobaltochelatase CbiXL: evidence for a 4Fe-4S center housed within an MXCXXC motif

CbiX is a cobaltochelatase required for the biosynthesis of vitamin B12 and is found in Archaea as a short form (CbiXS containing 120-145 amino acids) and in some bacteria as a longer version (CbiXL containing 300-350 amino acids). Purification of either recombinant Bacillus megaterium or Synechocystis CbiXL in Escherichia coli, which is facilitated by the presence of a naturally occurring histidine-rich region of the protein, results in the isolation of a dark brown protein solution. The UV/visible spectrum of the protein is consistent with the presence of a redox group, and the lack of definition within the spectrum is suggestive of a 4Fe-4S center. The presence of an iron-sulfur center was confirmed by EPR analysis of the proteins, which produces a pseudoaxial spectrum with g values at 2.04, 1.94, and 1.90. The EPR spectrum was absent at 70 K, an observation that is diagnostic of a 4Fe-4S center. Redox potentiometry coupled with optical spectroscopy allowed the midpoint potential of the redox center to be determined for the CbiXL from both B. megaterium and Synechocystis. Sequence analysis of CbiXL proteins reveals only two conserved cysteine residues within the CbiXL proteins, which are part of an MXCXXC motif. Mutagenesis of the two cysteines leads to loss of both the EPR spectrum and UV/visible spectral features of the Fe-S center in the protein, clearly indicating that these residues are involved in ligating the cofactor to the apoprotein possibly in a butterfly arrangement. The potential physiological role of the iron-sulfur center is discussed.

A story of chelatase evolution: identification and characterization of a small 13-15-kDa "ancestral" cobaltochelatase (CbiXS) in the archaea

The cobaltochelatase required for the synthesis of vitamin B12 (cobalamin) in the archaeal kingdom has been identified as CbiX through similarity searching with the CbiX from Bacillus megaterium. However, the CbiX proteins in the archaea are much shorter than the CbiX proteins found in eubacteria, typically containing less than half the number of amino acids in their primary structure. For this reason the shorter CbiX proteins have been termed CbiXS and the longer versions CbiXL. The CbiXS proteins from Methanosarcina barkeri and Methanobacter thermoautotrophicum were overproduced in Escherichia coli as recombinant proteins and characterized. Through complementation studies of a defined chelatase-deficient strain of E. coli and by direct in vitro assays the function of CbiXS as a sirohydrochlorin cobaltochelatase has been demonstrated. On the basis of sequence alignments and conserved active site residues we suggest that CbiXS may represent a primordial chelatase, giving rise to larger chelatases such as CbiXL, SirB, CbiK, and HemH through gene duplication and subsequent variation and selection. A classification scheme for chelatases is proposed.

Identification and functional analysis of enzymes required for precorrin-2 dehydrogenation and metal ion insertion in the biosynthesis of sirohaem and cobalamin in Bacillus megaterium

In Bacillus megaterium, the hemAXBCDL genes were isolated and were found to be highly similar to the genes from Bacillus subtilis that are required for the conversion of glutamyl-tRNA into uroporphyrinogen III. Overproduction and purification of HemC (porphobilinogen deaminase) and -D (uroporphyrinogen III synthase) allowed these enzymes to be used for the in vitro synthesis of uroporphyrinogen III from porphobilinogen. A second smaller cluster of three genes (termed sirABC) was also isolated and found to encode the enzymes that catalyse the transformation of uroporphyrinogen III into sirohaem on the basis of their ability to complement a defined Escherichia coli (cysG) mutant. The functions of SirC and -B were investigated by direct enzyme assay, where SirC was found to act as a precorrin-2 dehydrogenase, generating sirohydrochlorin, and SirB was found to act as a ferrochelatase responsible for the final step in sirohaem synthesis. CbiX, a protein found encoded within the main B. megaterium cobalamin biosynthetic operon, shares a high degree of similarity with SirB and acts as the cobaltochelatase associated with cobalamin biosynthesis by inserting cobalt into sirohydrochlorin. CbiX contains an unusual histidine-rich region in the C-terminal portion of the protein, which was not found to be essential in the chelation process. Sequence alignments suggest that SirB and CbiX share a similar active site to the cobaltochelatase, CbiK, from Salmonella enterica.

The biosynthesis of adenosylcobalamin (vitamin B12)

Vitamin B12, or cobalamin, is one of the most structurally complex small molecules made in Nature. Major progress has been made over the past decade in understanding how this synthesis is accomplished. This review covers some of the most important findings that have been made and provides the reader with a complete description of the transformation of uroporphyrinogen III into adenosylcobalamin (AdoCbl). 183 references are cited.

Structural diversity in metal ion chelation and the structure of uroporphyrinogen III synthase

All tetrapyrroles are synthesized through a branched pathway, and although each tetrapyrrole receives unique modifications around the ring periphery, they all share the unifying feature of a central metal ion. Each pathway maintains a unique metal ion chelatase, and several tertiary structures have been determined, including those of the protoporphyrin ferrochelatase from both human and Bacillus subtilus, and the cobalt chelatase CbiK. These enzymes exhibit strong structural similarity and appear to function by a similar mechanism. Met8p, from Saccharomyces cerevisiae, catalyses ferrochelation during the synthesis of sirohaem, and the structure reveals a novel chelatase architecture whereby both ferrochelation and NAD(+)-dependent dehydrogenation take place in a single bifunctional active site. Asp-141 appears to participate in both catalytic reactions. The final common biosynthetic step in tetrapyrrole biosynthesis is the generation of uroporphyrinogen by uroporphyrinogen III synthase, whereby the D ring of hydroxymethylbilane is flipped during ring closure to generate the asymmetrical structure of uroporphyrinogen III. The recently derived structure of uroporphyrinogen III synthase reveals a bi-lobed structure in which the active site lies between the domains.

Isolation and characterization of 14 additional genes specifying the anaerobic biosynthesis of cobalamin (vitamin B12) in Propionibacterium freudenreichii (P. shermanii)

A search for genes encoding enzymes involved in cobalamin (vitamin B12) production in the commercially important organism Propionibacterium freudenreichii (P. shermanii) has resulted in the isolation of an additional 14 genes encoding enzymes responsible for 17 steps of the anaerobic B12 pathway in this organism. All of the genes believed to be necessary for the biosynthesis of adenosylcobinamide from uroporphyrinogen III have now been isolated except two (cbiA and an as yet unidentified gene encoding cobalt reductase). Most of the genes are contained in two divergent operons, one of which, in turn, is closely linked to the operon encoding the B12-dependent enzyme methylmalonyl-CoA mutase. The close linkage of the three genes encoding the subunits of transcarboxylase to the hemYHBXRL gene cluster is reported. The functions of the P. freudenreichii B12 pathway genes are discussed, and a mechanism for the regulation of cobalamin and propionic acid production by oxygen in this organism is proposed.

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.

Optimization of Met8p crystals through protein-storage buffer manipulation

Sirohaem, the prosthetic group of assimilatory sulfite and nitrite reductases, is a modified tetrapyrrole that belongs to the same fraternity of metallo-prosthetic groups as haem, chlorophyll, cobalamin and coenzyme F430 [Warren & Scott (1990), Trends Biochem Sci. 15, 486-491]. In Saccharomyces cerevisiae, the last step in the biosynthesis of sirohaem involves Met8p, a bifunctional enzyme responsible for both the NAD(+)-dependent dehydrogenation of the corrin ring and ferrochelation. Optimization of the protein storage buffer according to the results of crystallization trials resulted in a more monodisperse protein solution. Crystals were grown that diffracted to 2.1 A.

The enigma of cobalamin (Vitamin B12) biosynthesis in Porphyromonas gingivalis. Identification and characterization of a functional corrin pathway

The ability of Porphyromonas gingivalis to biosynthesize tetrapyrroles de novo has been investigated. Extracts of the bacterium do not possess activity for 5- aminolevulinic-acid dehydratase or porphobilinogen deaminase, two key enzymes involved in the synthesis of uroporphyrinogen III. Similarly, it was not possible to detect any genetic evidence for these early enzymes with the use of degenerate polymerase chain reaction. However, the bacterium does appear to harbor some of the enzymes for cobalamin biosynthesis since cobyric acid, a pathway intermediate, was converted into cobinamide. Furthermore, degenerate polymerase chain reaction with primers to cbiP, which encodes cobyric-acid synthase, produced a fragment with a high degree of identity to Salmonella typhimurium cbiP. Indeed, the recently released genome sequence data confirmed the presence of cbiP together with 14 other genes of the cobalamin pathway. A number of these genes were cloned and functionally characterized. Although P. gingivalis harbors all the genes necessary to convert precorrin-2 into cobalamin, it is missing the genes for the synthesis of precorrin-2. Either the organism has a novel pathway for the synthesis of precorrin-2, or more likely, it has lost this early part of the pathway. The remainder of the pathway may be being maintained to act as a salvage route for corrin synthesis.

Biosynthesis of cobalamin (vitamin B12): a bacterial conundrum

The biosynthesis of cobalamin (vitamin B12) is described, revealing how the concerted action of around 30 enzyme-mediated steps results in the synthesis of one of Nature's most structurally complex 'small molecules'. The plethora of genome sequences has meant that bacteria capable of cobalamin synthesis can be easily identified and their biosynthetic genes compared. Whereas only a few years ago cobalamin synthesis was thought to occur by one of two routes, there are apparently a number of variations on these two pathways, where the major differences seem to be concerned with the process of ring contraction. A comparison of what is currently known about these pathways is presented. Finally, the process of cobalt chelation is discussed and the structure/function of the cobalt chelatase associated with the oxygen-independent pathway (CbiK) is described.

Common chelatase design in the branched tetrapyrrole pathways of heme and anaerobic cobalamin synthesis

Prosthetic groups such as heme, chlorophyll, and cobalamin (vitamin B(12)) are characterized by their branched biosynthetic pathway and unique metal insertion steps. The metal ion chelatases can be broadly classed either as single-subunit ATP-independent enzymes, such as the anaerobic cobalt chelatase and the protoporphyrin IX (PPIX) ferrochelatase, or as heterotrimeric, ATP-dependent enzymes, such as the Mg chelatase involved in chlorophyll biosynthesis. The X-ray structure of the anaerobic cobalt chelatase from Salmonella typhimurium, CbiK, has been solved to 2.4 A resolution. Despite a lack of significant amino acid sequence similarity, the protein structure is homologous to that of Bacillus subtilis PPIX ferrochelatase. Both enzymes contain a histidine residue previously identified as the metal ion ligand, but CbiK contains a second histidine in place of the glutamic acid residue identified as a general base in PPIX ferrochelatase. Site-directed mutagenesis has confirmed a role for this histidine and a nearby glutamic acid in cobalt binding, modulating metal ion specificity as well as catalytic efficiency. Contrary to the predicted protoporphyrin binding site in PPIX ferrochelatase, the precorrin-2 binding site in CbiK is clearly defined within a large horizontal cleft between the N- and C-terminal domains. The structural similarity has implications for the understanding of the evolution of this branched biosynthetic pathway.

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.

Cobalamin (vitamin B12) biosynthesis: functional characterization of the Bacillus megaterium cbi genes required to convert uroporphyrinogen III into cobyrinic acid a,c-diamide

The function of individual genes of the Bacillus megaterium cobI operon genes in cobalamin (vitamin B12) biosynthesis was investigated by their ability to complement defined Salmonella typhimurium cob mutants. This strategy confirmed the role of cbiA, -D, -F, -J, -L and cysGA. Furthermore the operon as a whole was used to restore corrin biosynthesis in Escherichia coli, which, although closely related to S. typhimurium, does not possess the CobI pathway. When the B. megaterium cob operon was cloned into a plasmid and transformed into an E. coli strain containing the S. typhimurium cbiP, it conferred upon the host strain the ability to make the cobyric acid de novo. However, cobyric acid synthesis was observed only when the strain was grown anaerobically. Derivatives of the corrin-producing E. coli strain were constructed in which genes of the B. megaterium cob operon had been inactivated. These strains were used to demonstrate that, whereas B. megaterium cbiD, -G and -X are essential for cobyric acid synthesis, the cbiW and -Y genes could be deleted without detriment to cobyric acid production in E. coli.

Cobalamin (vitamin B12) biosynthesis: identification and characterization of a Bacillus megaterium cobI operon

A 16 kb DNA fragment has been isolated from a Bacillus megaterium genomic library and fully sequenced. The fragment contains 15 open reading frames, 14 of which are thought to constitute a B. megaterium cobalamin biosynthetic (cob) operon. Within the operon, 11 genes display similarity to previously identified Salmonella typhimurium cobalamin biosynthetic genes (cbiH60, -J, -C, -D, -ET, -L, -F, -G, -A, cysGA and btuR), whereas three do not (cbiW, -X and -Y). The genes of the B. megaterium cob operon were compared with the cobalamin biosynthetic genes of Pseudomonas denitrificans, Methanococcus jannaschii and Synechocystis sp. Taking into account the presence of cbiD and cbiG, the absence of a cobF, cobG and cobN, -S and -T, it was concluded that B. megaterium, M. jannaschii and Synechocystis sp., like S. typhimurium, synthesize cobalamin by an anaerobic pathway, in which cobalt is added at an early stage and molecular oxygen is not required.

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.

Cobalamin (vitamin B12) biosynthesis--cloning, expression and crystallisation of the Bacillus megaterium S-adenosyl-L-methionine-dependent cobalt-precorrin-4 transmethylase CbiF

The Bacillus megaterium cbiF, encoding the cobalt-precorrin-4 S-adenosyl-L-methionine-dependent transmethylase of the anaerobic cobalamin biosynthetic pathway, has been cloned and overexpressed as a His-tagged recombinant protein in Escherichia coli. The protein was purified to homogeneity by a combination of metal chelate chromatography and high-resolution anion-exchange chromatography. The protein migrated with a subunit mass of 31 kDa by SDS/PAGE and with a molecular mass of 62 kDa by analytical gel filtration, suggesting that the native recombinant protein is a homodimer. The His-tagged protein was physiologically active as it was able to complement a Salmonella typhimurium cbiF mutant. However, the protein did not bind S-adenosyl-L-methionine with the same avidity as observed with other corrin biosynthetic transmethylases. A crystallisation screen of the purified protein led to the identification of two discrete crystal forms. One of these forms has been characterised and a full data set collected.

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

The Escherichia coli CysG protein (sirohaem synthase) catalyses four separate reactions that are required for the transformation of uroporphyrinogen III into sirohaem, initially two S-adenosyl-l-methionine-dependent transmethylations at positions 2 and 7, mediated through the C-terminal, or CysGA, catalytic domain of the protein, and subsequently a ferrochelation and dehydrogenation, mediated through the N-terminal, or CysGB, catalytic domain of the enzyme. This report describes how the deletion of the NAD+-binding site of CysG, located within the first 35 residues of the N-terminus, is detrimental to the activity of CysGB but does not affect the catalytic activity of CysGA, whereas the mutation of a number of phylogenetically conserved residues within CysGA is detrimental to the transmethylation reaction but does not affect the activity of CysGB. Further studies have shown that CysGB is not essential for cobalamin biosynthesis because the presence of the Salmonella typhimurium CobI operon with either cysGA or the Pseudomonas denitrificans cobA are sufficient for the synthesis of cobyric acid in an E. coli cysG deletion strain. Evidence is also presented to suggest that a gene within the S. typhimurium CobI operon might act as a chelatase that, at low levels of cobalt, is able to aid in the synthesis of sirohaem.

A role for Salmonella typhimurium cbiK in cobalamin (vitamin B12) and siroheme biosynthesis

The role of cbiK, a gene found encoded within the Salmonella typhimurium cob operon, has been investigated by studying its in vivo function in Escherichia coli. First, it was found that cbiK is not required for cobalamin biosynthesis in the presence of a genomic cysG gene (encoding siroheme synthase) background. Second, in the absence of a genomic cysG gene, cobalamin biosynthesis in E. coli was found to be dependent upon the presence of cobA(P. denitrificans) (encoding the uroporphyrinogen III methyltransferase from Pseudomonas denitrificans) and cbiK. Third, complementation of the cysteine auxotrophy of the E. coli cysG deletion strain 302delta a could be attained by the combined presence of cobA(P. denitrificans) and the S. typhimurium cbiK gene. Collectively these results suggest that CbiK can function in fashion analogous to that of the N-terminal domain of CysG (CysG(B)), which catalyzes the final two steps in siroheme synthesis, i.e., NAD-dependent dehydrogenation of precorrin-2 to sirohydrochlorin and ferrochelation. Thus, phenotypically CysG(B) and CbiK have very similar properties in vivo, although the two proteins do not have any sequence similarity. In comparison to CysG, CbiK appears to have a greater affinity for Co2+ than for Fe2+, and it is likely that cbiK encodes an enzyme whose primary role is that of a cobalt chelatase in corrin biosynthesis.

Salmonella typhimurium cobalamin (vitamin B12) biosynthetic genes: functional studies in S. typhimurium and Escherichia coli

In order to study the Salmonella typhimurium cobalamin biosynthetic pathway, the S. typhimurium cob operon was isolated and cloned into Escherichia coli. This approach has given the new host of the cob operon the ability to make cobalamins de novo, an ability that had probably been lost by this organism. In total, 20 genes of the S. typhimurium cob operon have been transferred into E. coli, and the resulting recombinant strains have been shown to produce up to 100 times more corrin than the parent S. typhimurium strain. These measurements have been performed with a quantitative cobalamin microbiological assay which is detailed in this work. As with S. typhimurium, cobalamin synthesis is only observed in the E. coli cobalamin-producing strains when they are grown under anaerobic conditions. Derivatives of the cobalamin-producing E. coli strains were constructed in which genes of the cob operon were inactivated. These strains, together with S. typhimurium cob mutants, have permitted the determination of the genes necessary for cobalamin production and classification of cbiD and cbiG as cobl genes. When grown in the absence of endogenous cobalt, the oxidized forms of precorrin-2 and precorrin-3, factor II and factor III, respectively, were found to accumulate in the cytosol of the corrin-producing E. coli. Together with the finding that S. typhimurium cbiL mutants are not complemented with the homologous Pseudomonas denitrificans gene, these results lend further credence to the theory that cobalt is required at an early stage in the biosynthesis of cobalamins in S. typhimurium.