Tagetitoxin: a new inhibitor of eukaryotic transcription by RNA polymerase III.

We demonstrate that tagetitoxin, a bacterial phytotoxin, preferentially inhibits eukaryotic RNA polymerase III. We used promoter-directed transcription of cloned genes in cell-free extracts to compare tagetitoxin inhibition of RNA polymerases from diverse sources. In HeLa cell extracts, accumulation of 5 S rRNA, and U6 snRNAs (transcribed by RNA polymerase III) was inhibited at 0.3-3.0 microM tagetitoxin but transcription from adenovirus 2 major late promoter (RNA polymerase II) was not significantly affected at concentrations below 30 microM. Tagetitoxin also inhibited promoter-directed RNA polymerase III transcription in cell-free extracts from Bombyx mori (pre-tRNA), and Saccharomyces cerevisiae (pre-tRNA) at 0.3-3.0 microM, concentrations that also inhibit chloroplast or bacterial promoter-directed transcription. In nonspecific transcription assays, partially purified B. mori RNA polymerase III was inhibited by tagetitoxin at concentrations that inhibit Escherichia coli RNA polymerase; purified calf thymus RNA polymerase II was not inhibited by tagetitoxin. Using injection into Xenopus laevis oocytes, we compared tagetitoxin effects on accumulation of U1 snRNA, hH2B mRNA (transcribed by RNA polymerase II), 5 S rRNA and U6 snRNA (RNA polymerase III), and 5.8 S rRNA (RNA polymerase I). In Xenopus oocytes, RNA polymerase III transcription was preferentially inhibited by tagetitoxin.