Tabtoxin, also known as wildfire toxin, is a simple

monobactam Monobactams are bacterially-produced monocyclic β-lactam antibiotics. The β-lactam ring is not fused to another ring, in contrast to most other β-lactams. Monobactams are narrow-spectrum antibiotics effective only against (strictly or fa ...

phytotoxin Phytotoxins are substances that are poisonous or toxic to the growth of plants. Phytotoxic substances may result from human activity, as with herbicides, or they may be produced by plants, by microorganisms, or by naturally occurring chemical react ...

produced by ''

Pseudomonas syringae ''Pseudomonas syringae'' is a rod-shaped, Gram-negative bacterium with polar flagella. As a plant pathology, plant pathogen, it can infect a wide range of species, and exists as over 50 different pathovars, all of which are available to research ...

''. It is the precursor to the

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy ...

tabtoxinine β-lactam (TBL). It is produced by: * Pseudomonas syringae pv. tabaci, the causal agent of the wildfire of tobacco. * P. syringae pv. coronafaciens * P. syringae pv. garcae * P. syringae BR2, causes a disease of bean (Phaseolus vulgaris) similar to tobacco wildfire. This organism is closely related to P. syringae pv. tabaci but cannot be classified in the pathovar tabaci because it is not pathogenic on tobacco. Tabtoxin is a dipeptide precursor to the biologically active form of TBL, differing by having an extra

threonine Threonine (symbol Thr or T) is an amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH form when dissolved in water), a carboxyl group (which is in the deprotonated −COO− ...

attached by a peptide bond to the C terminus. Tabtoxin is required by BR2(R) for both

chlorosis In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...

and lesion formation on bean. All mutations that affected tabtoxin production, whether spontaneous deletion or transposon induced, also affected lesion formation, and in all cases, restoration of tabtoxin production also restored pathogenic symptoms. Other factors may be required for BR2 to be pathogenic on bean, but apparently these are in addition to tabtoxin production. TBL functions as a toxin by inhibition of

glutamine synthetase Glutamine synthetase (GS) () is an enzyme that catalyzes the condensation of glutamate and ammonia to form glutamine: Glutamate + ATP + NH3 → Glutamine + ADP + phosphate Glutamine synthetase uses ammonia produced by nitrate reduction ...

Self-resistance

Tabtoxin resistance protein (TTR) is an enzyme that catalyzes the acetylation of TBL, rendering tabtoxin-producing pathogens tolerant to their own phytotoxins. The structure of an inactivated mutant of TTR is solved with its natural cofactor acetyl-CoA to 1.55 Å resolution. The binary complex forms a characteristic “V” shape for substrate binding and contains the four motifs conserved in the GCN5-related N-acetyltransferase (GNAT) superfamily, which also includes the

histone acetyltransferase Histone acetyltransferases (HATs) are enzymes that acetylation, acetylate conserved lysine amino acids on histone proteins by transferring an acetyl group from acetyl-CoA to form ε-N-acetyllysine, ε-''N''-acetyllysine. DNA is wrapped around his ...

s (HATs). There are reports that TTR possesses HAT activity and suggest an evolutionary relationship between TTR and other GNAT members. The production of tabtoxin itself is also part of the self-resistance strategy. These pathogens produce TBL before tabtoxin is produced; to detoxify, the enzyme TblF links TBL to threonine (Thr), producing a non-toxic product.

Biosynthesis

The biosynthetic precursors of tabtoxin were identified by the incorporation of 13C-labeled compounds. L-threonine and L-aspartate make up the side chain, while pyruvic acid and the methyl group of L-methionine make up β-lactam moiety. A biosynthetic model for the formation of TβL resembles that of

lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. Lysine contains an α-amino group (which is in the protonated form when the lysine is dissolved in water at physiological pH), an α-carboxylic acid group ( ...

, where the first dedicated step is the DapA-catalyzed condensation of aspartic acid semialdehyde with pyruvate to form L-2,3-dihydropicolinate (DHDPA). Tabtoxin biosynthesis branches off from the lysine biosynthetic pathway before the formation of

diaminopimelate Diaminopimelic acid (DAP) is an amino acid, representing an epsilon-carboxy derivative of lysine. ''meso''-α,ε-Diaminopimelic acid is the last intermediate in the biosynthesis of lysine and undergoes decarboxylation by diaminopimelate decarbox ...

(DAP). The 31-kb biosynthetic cluster consists of: * TabA (), an enzyme related to lysA (

diaminopimelate decarboxylase The enzyme diaminopimelate decarboxylase () catalyzes the cleavage of carbon-carbon bonds in ''meso''- 2,6-diaminoheptanedioate (diaminopimelate) to produce CO2 and L-lysine, the essential amino acid. It employs the cofactor pyridoxal phospha ...

) * TabB (), an enzyme related to dapD ( THDPA succinyl-CoA succinyltransferase, THDPA-ST) * TblA (), an enzyme with no close paralogs (identified as a member of SAMe-dependent

methyltransferase Methyltransferases are a large group of enzymes that all methylate their substrates but can be split into several subclasses based on their structural features. The most common class of methyltransferases is class I, all of which contain a Ro ...

superfamily by InterPro) This pathway produces TBL; the enzyme TblF finalize the synthesis by linking TBL to Thr to form tabtoxin.

Factors affecting production

The effects of carbon, nitrogen sources and amino acids on growth and tabtoxin production by pv. tabaci, were examined by varying the components of a defined basal medium, which contained the following nutrients per liter: sucrose (10 g), KNO₃ (5 g), MgSO₄.7H₂O (0.2 g), CaCl₂.2H₂O (0.11 g), FeSO₄.7H₂O (20 mg), NaH₂PO₄.2H₂O (0.9 g) and H₂PO₄.3H₂O (1 g). Both growth and quantity of tabtoxin synthesized were significantly affected by carbon source, nitrogen source and amino acid supplements. Sorbitol, xylose and sucrose proved to be the best carbon sources for tabtoxin production. Specific toxin production was very low using glucose as a single carbohydrate source, although bacterial growth was well supported by glucose. Amount and type of nitrogen sources (NH₄Cl or KNO₃) affected the growth of pv. tabaci and quantities of tabtoxin produced. Nitrate is the best of these two forms of nitrogen for production of tabtoxin. Some progress has been made on elucidating factors that regulate tabtoxin biosynthesis in P. syringae.

Bioactivation

Zinc was shown to be required for the aminopeptidase activity, which hydrolyzes tabtoxin to release TβL.

References

* http://aem.asm.org/content/79/16/5023.long * {{cite journal , doi=10.1016/S0022-2836(02)01284-6 , pmid=12527305 , volume=325 , issue=5 , title=Crystal Structure of Tabtoxin Resistance Protein Complexed with Acetyl Coenzyme A Reveals the Mechanism for β-Lactam Acetylation , journal=Journal of Molecular Biology , pages=1019–1030, year=2003 , last1=He , first1=Hongzhen , last2=Ding , first2=Yi , last3=Bartlam , first3=Mark , last4=Sun , first4=Fei , last5=Le , first5=Yi , last6=Qin , first6=Xincheng , last7=Tang , first7=Hong , last8=Zhang , first8=Rongguang , last9=Joachimiak , first9=Andrzej , last10=Liu , first10=Jinyuan , last11=Zhao , first11=Nanming , last12=Rao , first12=Zihe Alpha-Amino acids Amino acid derivatives Bacterial toxins Lactams Azetidines