PHOSPHORYLATION OF RNA-POLYMERASE-II C-TERMINAL DOMAIN AND TRANSCRIPTIONAL ELONGATION

THE carboxy-terminal domain (CTD) of the large subunit of RNA polymera se II is essential in vivo(1-4), and is found in either an unphosphory lated (IIa) or hyperphosphorylated (IIo) form(5,6). The Drosophila uni nduced hsp70 and hsp26 genes, and the constitutively expressed beta-1 tubulin and Gapdh-2 genes, contain an RNA polymerase II complex which pauses after synthesizing a short transcript(7-10). We report here tha t, using an in vivo ultraviolet crosslinking technique(11) and antibod ies directed against the IIa and IIo forms of the CTD12, these paused polymerases have an unphosphorylated CTD. For genes containing a 5' pa used polymerase, passage of the paused RNA polymerase into an elongati onally competent mode in vivo coincides with phosphorylation of the CT D. Also, the level of phosphorylation of the CTD of elongating polymer ases is shown not to be related to the level of transcription, but is promoter specific.