Light and Internal Clock Regulate Fly Genes

This study appeared in the June 27, 2002, online issue of the Proceedings of the National Academy of Sciences.

Earlier studies had shown that the products of eight different genes are essential to the operation of the internal clock that influences the behavior and physiology of living creatures. It was presumed that these genes, in turn, somehow influence the expression of other genes throughout the body in order to control the timing of behaviors like sleep and wakefulness.

In the present study, researchers from Washington University School of Medicine (St. Louis, MO, USA) used DNA microarrays to measure the expression levels of nearly 14,000 genes (practically the complete Drosophila genome) at various time points during the day in normal flies and in flies missing one of the clock genes, called period. All flies were exposed to light for 12 hours, followed by dark for 12 hours. The cycle continued for a total of 96 hours. Genetic analyses were performed on half of the flies at six different time points on the fifth day.

The remaining flies were transferred into complete darkness for 48 hours. On the third day of darkness, gene expression was measured at the same six time points. This approach was chosen to detect genetic changes regulated by the internal circadian timekeeping system rather than by external cues.

Overall, the researchers obtained more than 70 readings for each of the nearly 14,000 genes, generating about a million individual measurements. Statistical analysis revealed that between 72 and 200 of the flies' 14,000 genes showed significant rhythms of gene expression in normal flies living in a daily light-dark cycle. Of these 72 genes, 22 continued to fluctuate when flies were collected after three days of complete darkness. This implies that the internal, circadian clock drives these 22 genes and not external cues such as light.

Mutant flies lacking the period gene were placed into the same two experimental conditions--light and dark fluctuations compared with complete darkness. The flies exposed to alternating light and dark still showed 18 genes with persistent, rhythmic oscillations, demonstrating that light and dark can directly drive rhythmic gene expression. An unanticipated finding was the discovery of hundreds of genes whose levels did not fluctuate with time of day but responded drastically to different lighting conditions or to the presence or absence of a circadian clock.





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