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A new role for histone tail modifications in transcription elongation.

RNA polymerase II (Pol II) faces a number of daunting tasks while transcribing genes. It must respond to a large array of transcription regulators, permitting distinct regulation of thousands of genes. It must coordinate its activities with RNA processing events and regulators of chromatin modifications and structure (Sims et al. 2004; Saunders et al. 2006). The C-terminal domain (CTD), a unique structure at the C terminus of Pol II’s largest subunit, permits coordination of these activities (Phatnani and Greenleaf 2006). The CTD consists of 26 (in yeast) to 52 (in mice and humans) copies of a heptapeptide motif, Y1S2P3T4S5P6S7, and is subject to phosphorylation and dephosphorylation during transcription. These modifications serve as spatial and temporal markers of Pol II’s progress through a cycle of gene transcription.