Answer:
Phosphorylation activation:
1- p53 (transcriptional activator)
2- CDK8-cyclin C (transcriptional inhibitor)
Explanation:
Phosphorylation is a well-studied post-translational modification mediated by a protein called 'kinases' which phosphorylates specific protein substrates. This post-translational modification (phosphorylation) acts as a reversible switch capable of modulating protein function. Phosphorylation at specific amino acid residues (e.g., tyrosine, serine) controls protein function by triggering conformational changes in the phosphorylated protein. For example, P53 is a transcription activator activated by phosphorylation at multiple sites in its N-terminal region. P53 activates transcription of different genes by modulating protein-protein interactions with different substrates (e.g., MDM-2, CBP/p300, TFII). On the other hand, cyclins are proteins that have no enzymatic activity but are able to activate cyclin-dependent kinases (CDKs) by binding to them. CDK8-cyclin C (SRB10-SRB11) is a negative regulator of transcription that is able to inhibit transcription by phosphorylating the carboxy-terminal domain of the largest subunit of RNA polymerase II (Pol II) before the formation of the preinitiation transcriptional complex.
