Final answer:
Cdk8, or Srb10, targets cyclin C. The full activation of a Cdk/cyclin complex requires binding and phosphorylation, allowing it to phosphorylate other proteins. Rb acts as a negative cell cycle regulator at the G₁ checkpoint by inhibiting transcription factors such as E2F until the cell has achieved the necessary size.
Step-by-step explanation:
The mediator kinase Cdk8, also known as Srb10, targets cyclin C as its regulatory partner. Cyclins regulate the cell cycle in conjunction with cyclin-dependent kinases (Cdks) when they are tightly bound together. For a Cdk to become fully active, the Cdk/cyclin complex must bind to a cyclin protein and then be phosphorylated by another kinase. The activation process is crucial as it enables the Cdk to phosphorylate other proteins, changing their shape and activating them to propel the cell past specific checkpoints in the cell cycle.
The protein Rb is a negative regulator of the cell cycle, acting at the G₁ checkpoint to prevent advancement of the cell cycle until the cell's size is sufficient. Rb achieves this by binding to transcription factors, particularly E2F. In its active, dephosphorylated state, Rb inhibits the production of proteins required for the G1/S transition by holding E2F. Phosphorylation of Rb relieves this inhibition, releasing E2F to initiate transcription and allow the cell to progress in the cycle.