Biosynthesis of biotin in microorganisms. VI. Further evidence for desthiobiotin as a precursor in Eschericia coli

Programming Surface Chemistry with Engineered Cells

We have developed synthetic gene networks that enable engineered cells to selectively program surface chemistry. E. coli were engineered to upregulate biotin synthase, and therefore biotin synthesis, upon biochemical induction. Additionally, two different functionalized surfaces were developed that utilized binding between biotin and streptavidin to regulate enzyme assembly on programmable surfaces. When combined, the interactions between engineered cells and surfaces demonstrated that synthetic biology can be used to engineer cells that selectively control and modify molecular assembly by exploiting surface chemistry. Our system is highly modular and has the potential to influence fields ranging from tissue engineering to drug development and delivery.

Biotin synthase: insights into radical-mediated carbon-sulfur bond formation

The enzyme cofactor and essential vitamin biotin is biosynthesized in bacteria, fungi, and plants through a pathway that culminates with the addition of a sulfur atom to generate the five-membered thiophane ring. The immediate precursor, dethiobiotin, has methylene and methyl groups at the C6 and C9 positions, respectively, and formation of a thioether bridging these carbon atoms requires cleavage of unactivated CH bonds. Biotin synthase is an S-adenosyl-l-methionine (SAM or AdoMet) radical enzyme that catalyzes reduction of the AdoMet sulfonium to produce 5'-deoxyadenosyl radicals, high-energy carbon radicals that can directly abstract hydrogen atoms from dethiobiotin. The available experimental and structural data suggest that a [2Fe-2S](2+) cluster bound deep within biotin synthase provides a sulfur atom that is added to dethiobiotin in a stepwise reaction, first at the C9 position to generate 9-mercaptodethiobiotin, and then at the C6 position to close the thiophane ring. The formation of sulfur-containing biomolecules through a radical reaction involving an iron-sulfur cluster is an unprecedented reaction in biochemistry; however, recent enzyme discoveries suggest that radical sulfur insertion reactions may be a distinct subgroup within the burgeoning Radical SAM superfamily. This article is part of a Special Issue entitled: Radical SAM enzymes and Radical Enzymology.

Uptake of extracellular biotin by Escherichia coli biotin prototrophs

Uptake of exogenous biotin by two Escherichia coli biotin prototroph strains, K-12 and Crookes, appeared to involve incorporation at a fixed number of binding sites located at the cell membrane. Incorporation was characterized as a binding process specific for biotin, not requiring energy, and stimulated by acidic pH. Constant saturation quantities of exogenous biotin were incorporated by these cells, and the amounts, which were titrated, depended on whether the cells were resting or dividing. Resting cells incorporated exogenous biotin amounting to 2% of their total intracellular biotin content. Fifty percent of the exogenous biotin was incorporated into their free biotin fraction, and 50% was incorporated into their bound biotin fraction. On the other hand, dividing cells incorporated exogenous biotin into all of their intracellular sites, 88% going into the intracellular-bound biotin fraction, and 12% going into the free biotin fraction. Calculations suggested that each cell contained approximately 3,000 binding sites for biotin. It was postulated that biotin incorporation sites might have been components of acetyl coenzyme A carboxylase located at or near the membrane.

Genetics of biotin biosynthesis in Bacillus subtilis

Biotin auxotrophs of Bacillus subtilis were isolated and classified into three groups according to growth requirements, cross-feeding pattern, and biotin precursors excreted into culture supernatant fluids. Mutant genes were mapped by transduction using phage PBS1. All presently identified bio genes were linked to aroG with an order of bio-aroG-argA-leu-1. No linked markers were found to the left of the bio loci.

Mutant of Escherichia coli with derepressed levels of the biotin biosynthetic enzymes

A cross-feeding technique was used to isolate a mutant of Escherichia coli K-12 that excretes 1,000 times more biotin into the growth medium than the parent strain. The mutant has high levels of the biotin biosynthetic enzymes even when grown in the presence of biotin. Desthiobiotin synthetase, the level of which was used as a measure of the biosynthetic activity of the biotin pathway, is not repressed by biotin at the concentration 250,000 times that sufficient to repress the enzyme in the wild type. The mutant gene is cotransducible with argC located at 77 min on the E. coli chromosome.

Deletion and complementation analysis of biotin gene cluster of Escherichia coli

Genetic deletions that terminate within the cluster of genes needed for biotin biosynthesis in Escherichia coli have been isolated and mapped by transduction with phages lambda and P1. These deletions order the point mutations in each of the five genes. Mutations causing biotin dependence were incorporated into lambdapbio transducing phages. New bio(-) mutations were induced by exposure of lambdapbio particles to ultraviolet light. Tests of complementation between such bio(-)pbio particles and bio(-) mutant cells divide the bio(-) mutations into five cistrons: bioA, bioB, bioF, bioC, and bioD. Certain bioA and bioF mutations exhibit intragenic complementation, suggesting that these genes determine enzymes composed of identical subunits.

Biosynthesis of biotin: synthesis of 7,8-diaminopelargonic acid in cell-free extracts of Escherichia coli

Cell-free extracts prepared from a biotin auxotroph of Escherichia coli were active in catalyzing the synthesis of 7,8-diaminopelargonic acid, an intermediate of the biotin pathway, from 7-oxo-8-aminopelargonic acid. The product was identified on the basis of its chromatographic characteristics and its biotin activities for biotin auxotrophs of E. coli. Enzyme activity was determined in a reaction coupled with the desthiobiotin synthetase system, which is required for the conversion of 7,8-diaminopelargonic acid to desthiobiotin, and by measuring the amount of desthiobiotin formed by microbiological assay. The reaction was stimulated by l-methionine and pyridoxal-5'-phosphate. l-Methionine could not be replaced by any other amino acids tested. Pyridoxamine and pyridoxamine-5'-phosphate were as active as pyridoxal phosphate. The enzyme, presumably an aminotransferase, was demonstrable in the parent strain of E. coli and all mutant strains tested with the exception of a strain which is able to grow on diaminopelargonic acid but not on 7-oxo-8-aminopelargonic acid. Furthermore, the enzyme was repressible by biotin. The results were consistent with the hypothesis that the biosynthesis of 7,8-diaminopelargonic acid from 7-oxo-8-aminopelargonic acid is an obligatory step in the biosynthetic pathway of biotin in E. coli.

Partial purification and properties of D-desthiobiotin synthetase from Escherichia coli

d-Desthiobiotin synthetase, an enzyme that catalyzes the synthesis of d-desthiobiotin from dl-7,8-diaminopelargonic acid and HCO(3) (-), was purified 100-fold from cells of a biotin mutant strain of Escherichia coli. Adenosine triphosphate and Mg(2+) were shown, especially in purified extracts, to be obligatory for enzyme activity, although concentrations higher than 5 mm caused severe inhibition of the reaction with unpurified cell-free extracts. Adenosine diphosphate and adenosine monophosphate were shown to inhibit the reaction, but fluoride (up to 50 mm) had no detectable effect. The product of the enzyme reaction was identical to d-desthiobiotin on the basis of biological activity and chromatography. Furthermore, when H(14)CO(3) (-) was used as a substrate, the radioactive product was shown to be (14)C-desthiobiotin labeled exclusively in the ureido carbon.

Synthesis of 7-oxo-8-aminopelargonic acid, a biotin vitamer, in cell-free extracts of Escherichia coli biotin auxotrophs

The enzymatic synthesis of 7-oxo-8-aminopelargonic acid (7-KAP) from pimelyl-coenzyme A and l-alanine was demonstrated in cell-free extracts of a biotin mutant of Escherichia coli K-12 which excretes only 7-KAP into the growth medium. This biotin vitamer was identified by its chromatographic and electrophoretic properties. The enzyme (7-KAP synthetase) was repressed when the organism was grown in biotin concentrations greater than 0.2 ng/ml. The parent strain and members of other mutant groups that excrete 7-KAP, in addition to other vitamers, also exhibited synthetase activity. A mutant group that failed to excrete 7-KAP was further sub-divided into three groups, one of which lacked synthetase activity. These results are discussed in relation to a previously proposed scheme for biotin biosynthesis in which the formation of 7-KAP is considered the point of entry for pimelic acid into the biotin pathway.

Genetic and biochemical analysis of the biotin loci of Escherichia coli K-12

Some 60 biotin auxotrophs of Escherichia coli K-12 were isolated and classified into four groups according to their cross-feeding patterns, excertion products, and their ability to show a growth response to various biotin vitamers. Since all the mutants could be transduced with lambdadbio phages known to carry the entire bioA locus, it was concluded that all of the mutation sites were located in this locus. It was also possible to derive a gene order for the different mutant groups on the basis of transduction studies with various lambdadbio phages that carry portions of the bioA locus. A possible biochemical pathway for the biosynthesis of biotin in E. coli K-12 is discussed.