Phosphorylation of the C-terminal Domain of RNA Polymerase II Plays Central Roles in the Integrated Events of Eucaryotic Gene Expression

Abstract

RNA polymerase II (Pol II) is the only polymerase to possess heptapeptide repeats in the C-terminal domain (CTD) of its large subunit. During transcription, CTD phopshorylation occurs and is maintained from initiation to termination. To date, among the three known CTD kinases possessing CDK-cyclin pairs, TFIIH is the only one that forms a preinitiation complex. The Mediator complex plays essential roles in transcription initiation and during the transition from initiation to elongation by transmitting signals from transcriptional activators to Pol II. P-TEFb specifically plays a role in transcription elongation. TFIIH and mediator phosphorylate serine 5 (Ser5) of the CTD heptapeptide repeat sequence, whereas P-TEFb phosphorylates serine 2 (Ser2). Recently, it has become clear that CTD phosphorylation is not only essential for transcription, but also as a platform for RNA processing and chromatin regulation. In this review, we discuss the central role of Pol II phosphorylation in these nuclear events.

Divergent transcription: A new feature of active promoters

“…63 Ser-2 phosphorylation is important for proper termination and pre-mRNA processing of gene transcripts. 64 The lack of Ser-2p dependent activities upstream of promoters leads us to believe that promoters contain information that leads to asymmetric delivery of the P-TEFb/ctk1 kinase (Fig. 2).…”

Transcriptional activators and activation mechanisms

“…It is known that the preinitiation complex undergoes several conformational changes before RNAP actually initiates transcription (Carey and Smale, 2000;Hirose and Ohkuma, 2007). For example, the promoter DNA is significantly bent and unwound upon TBP binding (Nikolov et al, 1992;Kim et al, 1993).…”

“…The number of heptapeptide repeats differs in different species; human RNAP has 52 such repeats whereas yeast RNAP has 26. The CTD, which is required for cell viability (Nonet et al, 1987), plays a critical role not only in transcription but also in RNA processing and chromatin regulation (Hirose and Ohkuma, 2007). Heptapeptide repeats are subject to extensive phosphorylation.…”

“…During preinitiation complex formation, the CTD is un-phosphorylated. It becomes phosphorylated at serine 5 (Ser5) when RNAP transitions from initiation to elongation, refereed to as promoter clearance or escape (Hirose and Ohkuma, 2007;Levine, 2011). This transition allows RNAP to gain competence to transcribe into the main body of the gene (Levine, 2011).…”

“…Thus, promoter clearance represents an important step toward productive gene transcription. It has been suggested that Ser5 phosphorylation, catalyzed by the Cdk7 kinase in the TFIIH complex and the Cdk8 kinase in the mediator complex, may itself be responsible for this transition (Hirose and Ohkuma, 2007). As RNAP transcribes into the main body of the gene, its CTD becomes phosphorylated at serine 2 (see below).…”