The CobII and CobIII regions of the cobalamin (vitamin B12) biosynthetic operon of Salmonella typhimurium

Escalante‐Semerena, Jorge C.; Johnson, Michael G.; Roth, John R.

doi:10.1128/jb.174.1.24-29.1992

Cited by 35 publications

(32 citation statements)

References 33 publications

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“…Several explanation have been proffered (11,17,48,49). In any model, the phosphoribosyltransferase reaction required for CobIII function must be performed by some other protein(s).…”

Section: Discussionmentioning

confidence: 99%

The cobalamin (coenzyme B12) biosynthetic genes of Escherichia coli

Lawrence

Roth

1995

J Bacteriol

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The enteric bacterium Escherichia coli synthesizes cobalamin (coenzyme B 12 ) only when provided with the complex intermediate cobinamide. Three cobalamin biosynthetic genes have been cloned from Escherichia coli K-12, and their nucleotide sequences have been determined. The three genes form an operon (cob) under the control of several promoters and are induced by cobinamide, a precursor of cobalamin. The cob operon of E. coli comprises the cobU gene, encoding the bifunctional cobinamide kinase-guanylyltransferase; the cobS gene, encoding cobalamin synthetase; and the cobT gene, encoding dimethylbenzimidazole phosphoribosyltransferase. The physiological roles of these sequences were verified by the isolation of Tn10 insertion mutations in the cobS and cobT genes. All genes were named after their Salmonella typhimurium homologs and are located at the corresponding positions on the E. coli genetic map. Although the nucleotide sequences of the Salmonella cob genes and the E. coli cob genes are homologous, they are too divergent to have been derived from an operon present in their most recent common ancestor. On the basis of comparisons of G؉C content, codon usage bias, dinucleotide frequencies, and patterns of synonymous and nonsynonymous substitutions, we conclude that the cob operon was introduced into the Salmonella genome from an exogenous source. The cob operon of E. coli may be related to cobalamin synthetic genes now found among non-Salmonella enteric bacteria.Although commonly touted as one of the largest and oldest of the cofactors participating in modern biochemistry (20,23,59,60), the physiological importance of cobalamin (coenzyme B 12 ), especially among enteric bacteria, has remained enigmatic (58, 63). Cobalamin is derived from uroporphyrinogen III, a precursor in the synthesis of heme, siroheme, and chlorophylls as well as cobamides (Fig. 1). The conversion of uroporphyrinogen III to the complex intermediate cobinamide is referred to as part I of the biosynthetic pathway (hereafter designated CobI). Part II of the pathway (CobII) entails the biosynthesis of dimethylbenzimidazole (DMB) from probable flavin precursors (11,27,39). The covalent linkage of cobinamide, DMB, and a phosphoribosyl group donated by nicotinate mononucleotide is achieved by part III of the pathway (CobIII). Although most enteric bacteria can synthesize cobalamin de novo under either aerobic or anaerobic conditions (38), Escherichia coli synthesizes cobalamin only when provided with the complex intermediate cobinamide (38, 66); therefore, E. coli performs only parts II and III of the cobalamin biosynthetic pathway.The cobalamin biosynthetic genes in the closely related enteric bacterium Salmonella typhimurium have been characterized, and most constitute a large, 20-gene operon located at min 41 on the genetic map (28, 29, 52). It has been proposed that these genes are not ancestral to the Salmonella chromosome but have been introduced by horizontal transfer at or following the divergence of the salmonellae from Escherichia speci...

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“…Several explanation have been proffered (11,17,48,49). In any model, the phosphoribosyltransferase reaction required for CobIII function must be performed by some other protein(s).…”

Section: Discussionmentioning

confidence: 99%

The cobalamin (coenzyme B12) biosynthetic genes of Escherichia coli

Lawrence

Roth

1995

J Bacteriol

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“…Strain JE50, which carries insertion cob-62::Mu dl1734(Kmr), served as donor in a transductional cross with JE1540 [cob-369 cob-236::TniOd(Tcr) cobA366:: Tnl0d(Cmr)] as recipient. The cob-369 mutation was 97% cotransducible with the cob-236: :Tnl0d(Tcr) marker, an insertion known to map in the downstream part of the CobI region of the cob operon (8).…”

Section: Methodsmentioning

confidence: 99%

“…All plasmids used in this study are listed in Table 1 and were derived from pJE2 (8). Plasmid pJO1 was generated by subcloning the approximately 3.4-kb S1 nuclease-blunted AatII fragment of pJE2 into the multiple cloning site of plasmid pSU21 (16), which had been previously digested with HincII.…”

Section: Methodsmentioning

confidence: 99%

cobU-dependent assimilation of nonadenosylated cobinamide in cobA mutants of Salmonella typhimurium

O’Toole

Escalante‐Semerena

1993

J Bacteriol

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The cobA locus of Salmonella typhimurium is involved in the assimilation of nonadenosylated cobinamide, (CN)2CBI, into cobalamin (CBL) under aerobic and anaerobic growth conditions. Aerobically, cobA mutants are unable to assimilate (CN)2CBI into CBL. However, under anaerobic conditions, cobA mutants assimilate (CN)2CBI into CBL as efficiently as cobA+ strains. On the basis of this observation, we postulated the existence of a cobA-independent pathway for the assimilation of (CN)2CBI into CBL that is functional under anaerobic growth conditions (J. C. Escalante-Semerena, S.-J. Suh, and J. R. Roth, J. Bacteriol. 172:273-280, 1990

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“…Genetic studies have shown that most of the B12 synthetic genes are located in the cob operon at minute 41 of the genetic map (13,20). The phenotypic defect of these mutants can be corrected by supplying exogenous cyanocobalamin (CN-B12).…”

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The control region of the pdu/cob regulon in Salmonella typhimurium

1994

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The pdu operon encodes proteins for the catabolism of 1,2-propanediol; the nearby cob operon encodes enzymes for the biosynthesis of adenosyl-cobalamin (vitamin B12), a cofactor required for the use of propanediol.These operons are transcribed divergently from distinct promoters separated by several kilobases. The regulation of the two operons is tightly integrated in that both require the positive activator protein PocR and both are subject to global control by the Crp and ArcA proteins. We have determined the DNA nucleotide sequences of the promoter-proximal portion of the pdu operon and the region between the pdu and cob operons. Four open reading frames have been identified, pduB, pduA, pduF, and pocR. The pduA and pduB genes are the first two genes of the pdu operon (transcribed clockwise). The pduA gene encodes a hydrophobic protein with 56% amino acid identity to a 10.9-kDa protein which serves as a component of the carboxysomes of several photosynthetic bacteria. The pduF gene encodes a hydrophobic protein with a strong similarity to the GIpF protein of Escherichia coli, which facilitates the diffusion of glycerol. The N-terminal end of the PduF protein includes a motif for a membrane lipoprotein-lipid attachment site as well as a motif characteristic of the MIP (major intrinsic protein) family of transmembrane channel proteins. We presume that the PduF protein facilitates the diffusion of propanediol. ThepocR gene encodes the positive regulatory protein of the cob andpdu operons and shares the helix-turn-helix DNA binding motif of the AraC family of regulatory proteins. The mutations cobR4 and cobR58 cause constitutive, pocR-independent expression of the cob operon under both aerobic and anaerobic conditions. Evidence that each mutation is a deletion creating a new promoter near the normal promoter site of the cob operon is presented.

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The CobII and CobIII regions of the cobalamin (vitamin B12) biosynthetic operon of Salmonella typhimurium

Cited by 35 publications

References 33 publications

The cobalamin (coenzyme B12) biosynthetic genes of Escherichia coli

The cobalamin (coenzyme B12) biosynthetic genes of Escherichia coli

cobU-dependent assimilation of nonadenosylated cobinamide in cobA mutants of Salmonella typhimurium

The control region of the pdu/cob regulon in Salmonella typhimurium

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