This article, of course, got my attention:
Clocking In And Out Of Gene Expression
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.
"It's built-in self destruction," said O'Malley. "It prevents you from activating a potent factor in the cells that just keeps the clock running and the gene continuing to be expressed." In that scenario, the result could be cancer, too much growth or an abnormal function.
"It means there's a fixed length of time that the molecule can work. When it's activated, it's already preprogrammed to be destroyed. The clock's running and each time an ubiquitin is added, it is another tick of the clock."
So, it is a five-step 'hourglass' timer of sorts, which I would not, for he fear of confusion, imprecisely call a 'clock'. A clock ends a cycle at the same state at which it begins, so the cycle can spontaneously repeat. An hourglass timer has a beginning and an end, and does not spontaneously restart the cycle.