INHIBITORS OF TRANSCRIPTION SUCH AS 5,6-DICHLORO-1-BETA-D-RIBOFURANOSYLBENZIMIDAZOLE AND ISOQUINOLINE SULFONAMIDE DERIVATIVES (H-8 AND H-7-ASTERISK) PROMOTE DEPHOSPHORYLATION OF THE CARBOXYL-TERMINAL DOMAIN OFRNA-POLYMERASE-II LARGEST SUBUNIT

The RNA polymerases IIO and IIA differ by the extent of phosphorylatio n in the carboxyl-terminal domain (CTD) of the largest subunit. It has been proposed that the IIA form of RNA polymerase II interacts with t he promoter to form a stable preinitiation complex whereas the IIO for m would be generated upon entry into initiation of transcription. Phos phorylation of the CTD might be required to release the interaction be tween the polymerase and the promoter binding factors. In this paper, we show that in the presence of actinomycin D, the phosphorylated IIO form accumulates. In contrast, the dephosphorylated IIA form accumulat es while the amount of phosphorylated IIo form decreases in cells trea ted with CTD-kinase inhibitors such as the nucleoside analog, 5,6-dich loro-1-beta-D-ribofuranosylbenzimidazole or the isoquinoline sulfonami de derivatives H-7 or H-8. These changes are fast and suggest a very rapid phosphate turnover on the CTD. Transcription is inhibited in int act cells by drug concentrations that are effective in altering CTD ph osphorylation, although no causal relationship is established yet. The se effects do not concern other cellular functions such as protein syn thesis, Thus isoquinoline sulfonamide derivatives might be helpful to further dissect the role of CTD phosphorylation in transcription.