Juglone, an inhibitor of the peptidyl-prolyl isomerase Pin1, also directlyblocks transcription

The C-terminal domain (CTD) of the large subunit of RNA polymerase II plays a role in transcription and RNA processing. Yeast ESS1, a peptidyl-prolyl cis/trans isomerase, is involved in RNA processing and can associate with t he CTD. Using several types of assays we could not find any evidence of an effect of Pin1, the human homolog of ESS1, on transcription by RNA polymera se II in vitro or on the expression of a reporter gene in vivo. However, an inhibitor of Pin1,5-hydroxy-1,4-naphthoquinone (juglone), blocked transcri ption by RNA polymerase II. Unlike N-ethyl-maleimide, which inhibited all p hases of transcription by RNA polymerase II, juglone disrupted the formatio n of functional preinitiation complexes by modifying sulfhydryl groups but did not have any significant effect on either initiation or elongation. Bot h RNA polymerases I and III, but not T7 RNA polymerase, were inhibited by j uglone. The primary target of juglone has not been unambiguously identified , although a site on the polymerase itself is suggested by inhibition of RN A polymerase II during factor-independent transcription of single-stranded DNA. Because of its unique inhibitory properties juglone should prove usefu l in studying transcription in vitro.