In the 1950s Jacques Monod and François Jacob of the Pasteur Institute in Paris showed that certain regulator proteins in Escherichia coli bacteria can repress the production of other proteins. Let's use the notation X -> Y to mean that protein X represses protein Y. If X goes up (that is, the protein appears), then Y will go down (the protein disappears) after a short time (say, one second). If X goes down and no other repressor for Y is up, then Y will go up one second after X goes down.
Now consider three proteins: A, B and C, such that A -> B -> C -> A. If A goes up, B goes down one second later. After another second, C goes up, and after one more second A goes back down. Then the pattern continues: B up, C down, A up, B down, and so on. We call this a circuit of proteins: A, B and C periodically appear and disappear, acting like a biochemical clock. Such a clock was actually constructed a few years ago by Michael Elowitz, then a graduate student at Princeton University, and his adviser Stanislas Leibler.
This article was originally published with the title Protein Chime.