The Phantom Mark: Enigmatic roles of phospho-Threonine 4 modification of the C-terminal domain of RNA polymerase II.

The largest subunit of RNA polymerase II (Pol II) has an unusual carboxyl-terminal domain (CTD). This domain is composed of a tandemly repeating heptapeptide, Y₁ S₂ P₃ T₄ S₅ P₆ S₇ , that has multiple roles in regulating Pol II function and processing newly synthesized RNA. Transient phosphorylation of Ser2 and Ser5 of the YS₂ PTS₅ PS repeat have well-defined roles in recruiting different protein complexes and coordinating sequential steps in gene transcription. As such, these phospho-marks encipher a molecular recognition code, colloquially termed the CTD code. In contrast, the contribution of phospho-Threonine 4 (pThr4/pT4) to the CTD code remains opaque and contentious. Fuelling the debate on the relevance of this mark to gene expression are the findings that replacing Thr4 with a valine or alanine has varied impact on cellular function in different species and independent proteomic analyses disagree on the relative abundance of pThr4 marks. Yet, substitution with negatively charged residues is lethal and even benign mutations selectively disrupt synthesis and 3' processing of distinct sets of coding and non-coding transcripts. Suggestive of non-canonical roles, pThr4 marked Pol II regulates distinct gene classes in a species- and signal-responsive manner. Hinting at undiscovered roles of this elusive mark, multiple signal-responsive kinases phosphorylate Thr4 at target genes. Here, we focus on this under-explored residue and postulate that the pThr4 mark is superimposed on the canonical CTD code to selectively regulate expression of targeted genes without perturbing genome-wide transcriptional processes. This article is categorized under: RNA Processing > 3' End Processing RNA Processing > Processing of Small RNAs RNA Processing > Splicing Regulation/Alternative Splicing.