PHOSPHORYLATION STATE OF THE RNA-POLYMERASE-II C-TERMINAL DOMAIN (CTD) IN HEAT-SHOCKED CELLS - POSSIBLE INVOLVEMENT OF THE STRESS-ACTIVATEDMITOGEN-ACTIVATED PROTEIN (MAP) KINASES

RNA polymerase (RNAP) II is a multisubunit enzyme composed of several different subunits. Phosphorylation of the C-terminal domain (CTD) of the largest subunit is tightly regulated. In quiescent or in exponenti ally growing cells, both the unphosphorylated (IIa) and the multiphosp horylated (IIo) subunits of RNAP II are found in equivalent amounts as the result of the equilibrated antagonist action of protein kinases a nd phosphatases. In Drosophila and mammalian cells, heat shock markedl y modifies the phosphorylation of the RNAP II CTD. Mild heat shocks re sult in dephosphorylation of the RNAP II CTD. This dephosphorylation i s blocked in the presence of actinomycin D, as the CTD dephosphorlyati on observed in the presence of protein kinase inhibitors. Thus, heat s hock might inactivate CTD kinases which are operative at normal growth temperatures, as some protein kinase inhibitors do. In contrast, seve re heat shocks are found to increase the amount of phosphorylated subu nit independently of the transcriptional activity of the cells. Mild a nd severe heat shocks activate protein kinases, which then phosphoryla te, in vitro and in vivo, the CTD fused to P-galactosidase. Most of th e heat-shock-activated CTD kinases present in cytosolic lysates co-pur ify with the activated mitogen-activated protein (MAP) kinases, p42(ma pk) and p44(mapk). The weak CTD kinase activation occurring upon mild heat shock might be insufficient to compensate for the heat inactivati on of the already existing CTD kinases. However, under severe stress, the MAP kinases are strongly heat activated and might prevail over the phosphatases. A survey of different cells and different heat-shock co nditions shows that the RNAP II CTD hyperphosphorylation rates follow the extent of MAP kinase activation. These observations lead to the pr oposal that the RNAP II CTD might be an in vivo target for the activat ed p42(mapk) and p44(mapk) MAP kinases.