In studies of breast cancer cells, [senior investigator Dr. Bert] O'Malley and his colleagues showed how the clock works. Using steroid receptor coactivator-3 (SRC-3), they demonstrated that activation requires addition of a phosphate molecule to the protein at one spot and addition of an ubiquitin molecule at another point. Each time the message of the gene is transcribed into a protein, another ubiquitin molecule is chained on. Five ubiquitins in the chain and the protein is automatically destroyed.A counter on a separate work tape: neat !
Main article at sciencedaily; link via Science and Reason.