What modification is necessary for promoter escape in eukaryotic transcription?

In eukaryotic transcription, promoter escape is a crucial step where RNA polymerase II (RNAP2) transitions from the initiation phase to elongation after synthesizing a short RNA transcript. For this transition to occur, the carboxy-terminal domain (CTD) of RNA polymerase II must undergo phosphorylation. This phosphorylation events primarily occur around the heptapeptide repeat sequence in the CTD, specifically the phosphorylation of serine residues.

When the CTD is phosphorylated, it facilitates the release of RNA polymerase II from certain transcription factors and promotes the recruitment of additional factors that are necessary for the elongation phase of transcription. This modification indicates that the polymerase is ready to proceed beyond the promoter and enter into productive elongation, allowing for the synthesis of a longer RNA transcript.

While other modifications like acetylation of chromatin, which can enhance transcriptional activity by making DNA more accessible, and methylation of the promoter region, which often represses transcription, play important roles in gene regulation, they do not directly facilitate the escape of the polymerase from the promoter. Instead, the phosphorylation of the RNAP2 tail is the specific modification required for transitioning from the initiation to the elongation stage of transcription.