Carl H. Johnson, PhD

Carl H. Johnson, PhD

Stevenson Professor, Biological Sciences

Professor, Molecular Physiology and Biophysics

U-8211 MCN (Learned Labs)
(615) 322-2384

Cellular and Molecular Biology of Biological Clocks

Research Description

Organisms and even single cells have endogenous biological "clocks" that allow them to tell the time of day. Research in our laboratory is directed towards understanding the cellular and molecular bases of these fascinating timing mechanisms in a variety of organisms: cyanobacteria ("blue-green algae"), plants, and animals. To analyze the molecular nature of the clock in the prokaryotic cyanobacteria, we have developed a bioluminescent reporter strain that expresses a daily rhythm of light emission. Using this bioluminescence rhythm as a marker, clock mutants have been identified. We found that the essential clock gene, KaiC, is rhythmically expressed and forms ATP-dependent hexamers. In collaboration with the laboratory of Dr. Martin Egli, we have crystallized KaiC to determine its three-dimensional structure and discover its phosphorylation sites. The three key bacterial clock proteins (KaiA + KaiB + KaiC) will show circadian oscillations in a test tube! In collaboration with the laboratories of Drs. Phoebe Stewart and Hassane Mchaourab, we are applying electron microscopic and biophysical methods to explain how these proteins oscillate in vitro. Furthermore, we are using clock mutants of cyanobacteria to provide the first rigorous evidence for the adaptive significance of circadian clocks in fitness.

We also study the neuroscience of the circadian system of mammals. To measure circadian rhythms in brain slices in vitro, we use transgenic mice that express luciferase rhythmically. As a model system, we have also created a fibroblast cell line that is stably transfected with a luciferase reporter and glows rhythmically. Therefore, we use luminescence as a tool to monitor circadian rhythms in the brain. Future studies will focus upon understanding the signal transduction pathway of calcium to the clock and the role of clock genes in the fundamental mammalian clockwork. 

We have recently extended our studies to the genetics of the human biological clock. We are examining clock gene polymorphisms in human populations to determine how the neurogenetics of the biological clock affects our ability to adapt to shiftwork cycles and how it can influence mental health (esp. depression). Because daily biological clocks control a myriad of fundamental cellular activities, including cell division, metabolism, gene expression, and ion channel, the elucidation of the timing mechanism will have ramifications for many aspects of temporal regulation, including mental health, cancer, and jet lag.

Finally, we have developed a new method for measuring protein-protein interactions based upon the resonance energy between a luciferase and a fluorescent protein. This method is called Bioluminescence Resonance Energy Transfer, or BRET. This technique has allowed us to develop novel reporters for intracellular calcium and hydrogen ions. We envision a bright future for this technique.