1 Tyr1 phosphorylation promotes phosphorylation of Ser2 on the C-terminal domain 2 of eukaryotic RNA polymerase II by P-TEFb 3 Joshua E. Mayfield1† *, Seema Irani2*, Edwin E. Escobar3, Zhao Zhang4, Nathanial T. 4 Burkholder1, Michelle R. Robinson3, M. Rachel Mehaffey3, Sarah N. Sipe3,Wanjie Yang1, 5 Nicholas A. Prescott1, Karan R. Kathuria1, Zhijie Liu4, Jennifer S. Brodbelt3, Yan Zhang1,5 6 1 Department of Molecular Biosciences, 2Department of Chemical Engineering, 7 3 Department of Chemistry and 5 Institute for Cellular and Molecular Biology, University of 8 Texas, Austin 9 4 Department of Molecular Medicine, Institute of Biotechnology, University of Texas 10 Health Science Center at San Antonio 11 †Current Address: Department of Pharmacology, University of California, San Diego, La 12 Jolla 13 * These authors contributed equally to this paper. 14 Correspondence: Yan Zhang (
[email protected]) 15 16 1 17 Summary 18 The Positive Transcription Elongation Factor b (P-TEFb) phosphorylates 19 Ser2 residues of C-terminal domain (CTD) of the largest subunit (RPB1) of RNA 20 polymerase II and is essential for the transition from transcription initiation to 21 elongation in vivo. Surprisingly, P-TEFb exhibits Ser5 phosphorylation activity in 22 vitro. The mechanism garnering Ser2 specificity to P-TEFb remains elusive and 23 hinders understanding of the transition from transcription initiation to elongation. 24 Through in vitro reconstruction of CTD phosphorylation, mass spectrometry 25 analysis, and chromatin immunoprecipitation sequencing (ChIP-seq) analysis, we 26 uncover a mechanism by which Tyr1 phosphorylation directs the kinase activity of 27 P-TEFb and alters its specificity from Ser5 to Ser2.