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 nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...

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 use ...

. This structure, however, has been questioned. The

absolute configuration Absolute configuration refers to the spatial arrangement of atoms within a chiral molecular entity (or group) and its resultant stereochemical description. Absolute configuration is typically relevant in organic molecules, where carbon is bonded ...

remains undetermined, and attempts at confirming the structure by

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

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 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 ...

. 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