Tabtoxin

Tabtoxin, also known as wildfire toxin, is a monobactam-type phytotoxin produced by Pseudomonas syringae. It is the precursor to the antibiotic 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, which 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 attached by a peptide bond to the C terminus. Tabtoxin is required by BR2(R) for both chlorosis 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. This toxin is a irreversible inhibitor of glutamine synthetase.

Tabtoxin, also known as wildfire toxin, is a monobactam-type phytotoxin produced by Pseudomonas syringae. It is the precursor to the antibiotic 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, which 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 attached by a peptide bond to the C terminus. Tabtoxin is required by BR2(R) for both chlorosis 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. This toxin is a irreversible inhibitor of glutamine synthetase.

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 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 acetyltransferases (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.

References

http://aem.asm.org/content/79/16/5023.long

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