Distinct roles for CTD Ser-2 and Ser-5 phosphorylation in the recruitment and allosteric activation of mammalian mRNA capping enzyme

Capping is targeted to pre-mRNAs through binding of the guanylyltransferase component of the capping apparatus to the phosphorylated CTD of RNA polyme rase II. We report that mammalian guanylyltransferase binds synthetic CTD p eptides containing phosphoserine at either position 2 or 5 of the YSPTSPS h eptad repeat. CTD peptides containing Ser-5-PO4 stimulate guanylyltransfera se activity by enhancing enzyme affinity for GTP and increasing the yield o f the enzyme-GMP intermediate. A CTD peptide containing Ser-2-PO4 has no ef fect on guanylyltransferase activity. This implies an allosteric change in guanylyltransferase conformation that is specified by the position of phosp hoserine in the CTD. Stimulation of guanylyltransferase increases with the number of Ser-5-phosphorylated heptads. Our results underscore how mRNA pro duction may be regulated by the display of different CTD phosphorylation ar rays during transcription elongation.