Suppression analysis reveals a functional difference between the serines in positions two and five in the consensus sequence of the C-terminal domain of yeast RNA polymerase II

The largest subunit of RNA polymerase II contains a repetitive C-terminal domain (CTD) consisting of tandem repeats of the consensus sequence Tyr₁Ser₂Pro₃Thr₄Ser₅Pro₆Ser₇. Substitution of nonphosphorylatable amino acids at positions two or five of the Saccharomyces cerevisiae CTD is lethal. We developed a selection system for isolating suppressors of this lethal phenotype and cloned a gene, SCA1 (suppressor of CTD alanine), which complements recessive suppressors of lethal multiple-substitution mutations. A partial deletion of SCA1 (sca1Δ::hisG) suppresses alanine or glutamate substitutions at position two of the consensus CTD sequence, and a lethal CTD truncation mutation, but SCA1 deletion does not suppress alanine or glutamate substitutions at position five. SCA1 is identical to SRB9, a suppressor of a cold-sensitive CTD truncation mutation. Strains carrying dominant SRB mutations have the same suppression properties as a sca1Δ::hisG strain. These results reveal a functional difference between positions two and five of the consensus CTD heptapeptide repeat. The ability of SCA1 and SRB mutant alleles to suppress CTD truncation mutations suggest that substitutions at position two, but not at position five, cause a defect in RNA polymerase II function similar to that introduced by CTD truncation.