Tagetitoxin (TGT) is a bacterial phytotoxin produced by ''Pseudomonas syringae pv. tagetis''.

Chemical structure

When TGT was first isolated, it was only partially characterized. The first proposed chemical structure of TGT involved an eight-membered ring, but this was revised shortly afterward to a bicyclic structure (shown at right) based on

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...

and

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...

. This structure, however, has been questioned. The

absolute configuration In chemistry, absolute configuration refers to the spatial arrangement of atoms within a molecular entity (or Functional group, group) that is chirality (chemistry), chiral, and its resultant stereochemical description. Absolute configuration is ...

remains undetermined, and attempts at confirming the structure by

organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...

are underway. Recently Porter et al. published a revised structure of TGT based on extensive 2D NMR data.

Mechanism of action

TGT interferes with development of chloroplasts in young plant leaves thereby causing chlorosis. The natural target of the toxin is chloroplast RNA polymerase. Chloroplast RNA polymerase belongs to ubiquitous family of multisubunit RNA polymerases (RNAP) and is most closely related to bacterial enzymes. ''In vitro'', TGT inhibits bacterial RNAPs from ''Escherichia coli'' and ''Thermus thermophilus'', and eukaryotic RNA polymerase III. In contrast, eukaryotic RNA polymerase I and II as well as single-subunit RNA polymerases of bacteriophage T7 and SP6 are relatively insensitive to the compound. TGT binds in the RNAP active site and inhibits initiation and elongation phases of transcription as well as pyrophosphorolysis of the nascent RNA. However, the detailed mechanism of inhibition remains a subject of heated debate. It has been suggested that TGT forms a ternary RNAP-NTP-TGT complex and inhibits phosphodiester bond synthesis either by binding an inhibitory magnesium ion or by trapping a flexible active site domain in an inactive conformation. The third theory suggests that TGT forms predominantly a binary RNAP-TGT complex and inhibits RNAP translocation along the DNA by mimicking the transcription byproduct pyrophosphate.

References

{{reflist, 30em Plant toxins Acetate esters Carboxylic acids Organophosphates RNA polymerase inhibitors Carboxamides Alcohols