Practice question

TFIIH includes CAK subcomplex cyclin activating kinase comprising CDK7 kinase, cyclin H, MAT1 assembly factor stabilizing kinase module. After promoter melting, CDK7 phosphorylates serine five residues within YSPTSPS heptad repeats of RPB1 C-terminal domain, converting initiation complex to early elongation state. Serine5 phosphorylation reduces affinity for Mediator, facilitating promoter escape, and creates binding site for capping enzymes and Set1 methyltransferase depositing H3K4 trimethylation. Later transcription factor P-TEFb containing CDK9 phosphorylates serine two to promote productive elongation and recruitment of polyadenylation and splicing machineries.