Phosphorylation of RNA polymerase II C-terminal domain and transcriptional elongation

Abstract
THE carboxy-terminal domain (CTD) of the large subunit of RNA polymerase II is essential in vivo1–4, and is found in either an unphosphorylated (Ha) or hyperphosphorylated (Ho) form5,6. The Drosophila uninduced hsp70 and hsp26 genes, and the constitutively expressed β-1 tubulin and Gapdh-2 genes, contain an RNA polymerase II complex which pauses after synthesizing a short transcript7–10. We report here that, using an in vivo ultraviolet crosslinking technique11 and antibodies directed against the I la and IIo forms of the CTD12, these paused polymerases have an unphosphorylated CTD. For genes containing a 5' paused polymerase, passage of the paused RNA polymerase into an elongationally competent mode in vivo coincides with phosphorylation of the CTD. Also, the level of phosphorylation of the CTD of elongating polymerases is shown not to be related to the level of transcription, but is promoter specific.