The very first item in this blog was about the circadian rhythm in cyanobacteria (It Don't Mean a Thing If It Ain't Got That Swing). We celebrated the amazing finding by a group from Nagoya University that the 24-hour circadian clock of Synechococcus elegans can be reconstituted in vitro with just three purified proteins in the presence of ATP. These proteins are kinases that interact with one another, carrying out phosphorylation reactions that exhibit the same periodicity as the whole cell. Mutants with altered clocks also show corresponding changes in vitro.
This group now reports that one of these proteins, the KaiC ATPase, is the timekeeper that is responsible for the characteristic oscillation. KaiC is not your average kinase — it hydrolyzes ATP at a snail's pace, about 15 molecules per day! And that activity peaks every 24 hours. In mutants with different day lengths, those with shorter days have higher KaiC ATPase activity; those with longer days show lower activity. Adaptively, the ticking of this ATPase clock is constant for temperatures from 25°C to 35°C. Putting this all together, they propose that "KaiC ATPase activity constitutes the most fundamental reaction underlying circadian periodicity in cyanobacteria...This finding represents the first description of a simple biochemical reaction acting as a circadian timekeeper."
A simple biochemical reaction it may be, but many questions remain. We look forward to the next chapter in this story.