GROWTH-RELATED CHANGES IN PHOSPHORYLATION OF YEAST RNA-POLYMERASE-II

The largest subunit of RNA polymerase II contains a unique C-terminal domain (CTD) consisting of tandem repeats of the consensus heptapeptid e sequence Tyr(1)-Ser(2)-Pro(3)-Thr(4)-Ser(5)Pro(6)-Ser(7). Two forms of the largest subunit can be separated by SDS-polyacrylamide gel elec trophoresis. The faster migrating form termed IIA contains little or n o phosphate on the CTD, whereas the slower migrating II0 form is multi ply phosphorylated. CTD kinases with different phosphoryl acceptor spe cificities are able to convert IIA to II0 in vitro, and different phos phoisomers have been identified in vivo. In this paper we report the b inding specificities of a set of monoclonal antibodies that recognize different phosphoepitopes on the CTD. Monoclonal antibodies like H5 re cognize phosphoserine in position 2, whereas monoclonal antibodies lik e H14 recognize phosphoserine in position 5. The relative abundance of these phosphoepitopes changes when growing yeast enter stationary pha se or are heat-shocked. These results indicate that phosphorylation of different CTD phosphoacceptor sites are independently regulated in re sponse to environmental signals.