Highlights d More than 100 possible combinations of various PTMs are identified in Rpb4/7 d PTMs change temporally during the various stages of Rpb4/ 7/mRNA life cycle d E-rich loop of Rpb4 involved in PTM-regulated binding of RNA processing factors d Mutations that mimic methylation of this E-rich loop affect mRNA buffering

Rpb4/7 binds RNA Polymerase II (Pol II) transcripts co-transcriptionally and accompanies them throughout their lives. By virtue of its capacity to interact with key regulators (e.g., Pol II, eIF3, Pat1) both temporarily and spatially, Rpb4/7 regulates the major stages of the mRNA lifecycle. Here we show that Rpb4/7 can undergo over 100 combinations of post-translational modifications (PTMs). Remarkably, the Rpb4/7PTMs repertoire changes as the mRNA/Rpb4/7 complex progresses from one stage to the next. A mutagenesis approach in residues that undergo PTMs suggests that temporal Rpb4 PTMs regulate its interactions with key regulators of gene expression that control transcriptional and post-transcriptional stages. Moreover, one mutant type specifically affects mRNA synthesis despite its normal association with Pol II, whereas the other affects both mRNA synthesis and decay; both types disrupt the balance between mRNA synthesis and decay ('mRNA buffering') and the cell's capacity to respond to the environment. Taken together, we propose that temporal Rpb4/7 PTMs are involved in cross talks among the various stages of the mRNA lifecycle.

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