Fine mapping of CC-3 and MPM-2 epitopes on Rpb1 consensus heptad repeats. (A) Biotinylated CTD peptides were immobilized on a streptavidin-coated ELISA plate at three different concentrations and incubated with MPM-2 and CC-3. These peptides contained four YSPTSPS repeats, phosphorylated either on the serine residues at position 2 (YS PTSPS), at position 5 (YSPTS PS) or at both positions (YS PTS PS). An unphosphorylated peptide was used as a control (YSPTSPS). CC-3 was mainly reactive with peptides phosphorylated on serine 2 of the consensus repeat whereas MPM-2 reacts preferentially with the peptide phosphorylated on both serines 2 and 5. Surprisingly, MPM-2 also reacted with the unphosphorylated peptide in a concentration independent manner (see text). (B) Histogram of the CC-3 and MPM-2 reactivities shown in (A).