Transcription-independent phosphorylation of the RNA polymerase IIC-terminal domain (CTD) involves ERK kinases (MEK1/2)

The largest subunit of the mammalian RNA polymerase II possesses a C-termin al domain (CTD) consisting of 52 repeats of the consensus sequence, Tyr(1)- Ser(2)-Pro(3)-Thr(4)-Ser(5)-Pro(6)-Ser(7). Phosphorylation of the CTD is kn own to play a key role in gene expression. We now show that treatments such as osmotic and oxidative shocks or serum stimulation generate a new type o f phosphorylated subunit, the IIm form. This IIm form might be generated in vivo by ERK-type MAP kinase phosphorylation as: (i) ERK1/2 are major CTD k inases found in cell extracts; (ii) the immunoreactivity of the IIm form ag ainst a panel of monoclonal antibodies indicates that the CTD is exclusivel y phosphorylated on Ser-5 in the repeats, like RNA polymerase II phosphoryl ated in vitro by an ERK1/2; and (iii) the IIm form does not appear when ERK activation is prevented by treating cells with low concentrations of highl y specific inhibitors of MEK1/2. Since the IIm subunit is not affected by i nhibition of transcription and is not bound to chromatin, it does not parti cipate in transcription.