FIG. 4.
Thr-338 phosphorylation is important for entry into stationary phase. (A) Growth rates of CTK1 and ctk1T338A cells. Yeast cultures were grown in YPD medium at 30°C, and cell density was measured during the transition from exponential to early stationary phase. The average of three independent experiments is presented. Similar results were obtained by direct counting of yeast cells. The time course started when cells reached an OD600 of 0.1. (B) Yeast extracts were prepared from CTK1 and ctk1T338A cells at different times during late exponential growth and early stationary phase. The indicated times are the same as in panel A. The proteins were resolved by SDS-PAGE and immunoblotted for the presence of Ser2-phosphorylated RNA pol II (Ser2-P) using H5 antibodies and for unphosphorylated RNA pol II (CTD) using 8WG16 antibodies. (C) Δcak1 cells were grown into early stationary phase and analyzed for the presence of RNA pol II isoforms as for panel B. WT, wild type.