CTD Tyrosine Phosphorylation Impairs Termination Factor Recruitment to RNA Polymerase II

Don't Terminate Me! DNA transcription progresses through three phases—initiation, elongation, and termination—of messenger RNA chains. The transcribing enzyme, RNA polymerase (Pol) II, recruits factors that assist in each of these phases. Mayer et al. (p. 1723) now show that the C-terminal domain (CTD) of actively elongating Pol II is phosphorylated at conserved tyrosine residues. This modification impairs recruitment of termination factors. Factor exchange on the transcribing polymerase enzyme may be explained by an extended CTD code that is based on differential phosphorylation of the tyrosines and two well-characterized serine residues in the CTD. Phosphorylation of a tyrosine inhibits the binding of termination factors and promotes the binding of elongation factors. In different phases of the transcription cycle, RNA polymerase (Pol) II recruits various factors via its C-terminal domain (CTD), which consists of conserved heptapeptide repeats with the sequence Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. We show that the CTD of transcribing yeast Pol II is phosphorylated at Tyr1, in addition to Ser2, Thr4, Ser5, and Ser7. Tyr1 phosphorylation stimulates binding of elongation factor Spt6 and impairs recruitment of termination factors Nrd1, Pcf11, and Rtt103. Tyr1 phosphorylation levels rise downstream of the transcription start site and decrease before the polyadenylation site, largely excluding termination factors from gene bodies. These results show that CTD modifications trigger and block factor recruitment and lead to an extended CTD code that explains transcription cycle coordination on the basis of differential phosphorylation of Tyr1, Ser2, and Ser5.

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