The mode of action of tagetitoxin

Tagetitoxin, a chlorosis-inducing phytotoxin produced by Pseudomonas syringae pv. tagetis, inhibits RNA synthesis directed by chloroplast RNA polymerase. The inhibition of chloroplast RNA synthesis appears to be a rapid and specific response to tagetitoxin. In isolated chloroplasts, addition of 100 μM tagetitoxin quickly reduced the incorporation of [3H]uridine into RNA but did not affect the incorporation of [35S]methionine into protein during the remaining incubation period. Incorporation of [3H]thymidine into DNA was only slight affected by the toxin, perhaps secondarily. Tagetitoxin inhibited the incorporation of [32P]UTP into RNA when added to transcriptionally-active chloroplast protein extracts. The toxin appears to inhibit RNA synthesis because it has no affect on RNA stability in the chloroplast extract. Tagetitoxin also inhibits RNA synthesis directed by the RNA polymerase from at least three different eubacteria; Escherichia coli, Anabaena 7120 and Pseudomonas syringae pv. tagetis