Composition of CTD CTD consists of tandem heptad repeats with the consensus sequence Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. This heptad sequence is highly conserved throughout all eukaryotes, with the number of repeats: 52x in mammals, 26x in S. cerevisiae, 29x in S. pombe, and 42x in Drosophila (Figure 2| ) (Chapman et al., 2008) . Most heptads follow the consensus sequence in yeast, while mammals show more deviations from the consensus sequence, with 21 consensus and 31 non-consensus repeats.

more »

... first 26 repeats are highly conserved, whereas the distal repeats are less conserved. The size of the CTD varies dramatically between species, with the lowest number being zero in Trypanosoma brucei and only five heptad repeats in Plasmodium yoelii. Between the CTD domain and Rpb1 main body is a linker region. The conservation of heptad repeats in the CTD region differs through species. For example, the CTD of yeast is highly conserved. In budding yeast, there are 7 out of 26 non-consensus repeats. In contrast, CTD of Drosophila contains only two consensus repeats. In humans, there are 21 consensus repeats of 52 (Figure 2| ).