The carboxy-terminal domain (CTD) of Rpb1, the largest component of the 12-subunit RNA polymerase II, consists of repeating Y1S2P3T4S5P6S7 heptapeptides (26 repeats in budding yeast). Each stage of transcription relies on the ordered recruitment and exchange of specific protein complexes that act on RNA polymerase II, its nascent transcripts, and the underlying chromatin. This dynamic process is orchestrated via patterned post-translational modifications of the CTD. To characterize the role of phosphorylation on Thr4, we examined the effect of Rpb1 alleles in which Thr4 was substituted with an alanine (T4A) or the phospho-mimic glutamate (T4E). Substitutions were made across all heptad repeats of the CTD. We affinity purified HA-tagged Rpb1 from Saccharomyces cerevisiae strains bearing WT, T4A, and T4E CTDs. A control strain (Z26) lacking the HA-tagged Rpb1 was subjected to an identical affinity enrichment procedure. Three biological replicates were acquired for each type of affinity purification and analyzed independently. After TCA-precipitation, proteins were urea-denatured, reduced, alkylated, then digested with endoproteinase LysC followed by trypsin. The resulting peptide mixtures were analyzed by Multidimensional Protein Identification Technology (MudPIT). Label-free quantitative proteomics was used to identify and quantify the relative abundance of affinity-enriched complexes.