The cyanobacterium Synechococcus elongatus organizes the timing of activities such as photosynthesis with an internal clock. The clock oscillates between methodically adding molecules to a specific protein and then removing them.


Video transcript

You might use an alarm clock to wake up, but the individual cells in your body also have clocks. Called circadian clocks, they operate in sync with the sun’s daily cycle, telling your body the optimal time to sleep, eat or do other biological jobs. Single-celled critters called cyanobacteria, which make food using energy from the sun, also have clocks that help them keep track of day and night. But how does their clock work?

Cyanobacteria can’t tick off time by counting one-one-thousand, two-one-thousand.... Their clock is made of just three proteins, all named Kai, short for the Japanese character “kaiten,” for “cycle.” KaiC is the clock centerpiece. It is shaped like two doughnuts stacked on each other.

At dawn, KaiC’s doughnuts are loosely stacked. Enter Kai Protein No. 2: KaiA. KaiA grabs on to little levers sticking out of KaiC, deforming its top doughnut. This prompts KaiC to slowly and methodically attach phosphates to itself. Tick-tock, tick-tock.

By dusk, KaiC’s top doughnut is stiff from being covered with phosphates. This makes the bottom doughnut bulge out. Enter Kai Protein No. 3: KaiB. KaiB zooms in, grabbing hold of the bottom doughnut. KaiA gets kicked off the top doughnut. With KaiA gone, KaiC begins to undo the day’s work, methodically removing all those phosphates. Tick-tock, tick-tock.

By dawn, KaiC has shed its phosphates. KaiC’s doughnuts relax; its little levers protrude again. The cycle starts over. But how does the clock guide cellular activity? When KaiC is decorating itself with phosphates, other proteins in the cell add phosphates to themselves as well. Some of these phosphate-wearing proteins can turn certain genes on and others off. The reverse is also true. Proteins without phosphates turn off the genes that were on and turn on the genes that were off.

The rhythmic oscillation of when the clock proteins are wearing phosphates — lots of them at dusk, but few at dawn — guide the cells’ metabolism, telling it what activities to do and when. Tick-tock!