Phosphorylation of Ser5 in CTD is associated with
Phosphorylation of CTD heptad serine five by TFIIH CDK7 occurs early during transcription, peaking within fifty base pairs downstream of promoter. Serine5-phosphorylated CTD serves as landing pad for capping enzyme complex containing RNA triphosphatase, guanylyltransferase, N7-methyltransferase activities, stimulating cotranscriptional cap addition when transcript reaches twenty to thirty nucleotides. Marker also recruits early spliceosome components and H3K4 methyltransferase Set1 complex via interaction with Paf1 elongation complex. Phosphatases Ssu72 and Rtr1 later dephosphorylate serine fi
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 6: Ser5-P associated with initiation and promoter escape; PLoS Biology CTD phosphorylation dynamics