Splices of Time

Tina Hesman Saey in Science News:

Fruit flies and plants have independently come up with similar ways to mark time, a new study suggests.

Both modify the products of certain genes based on daily rhythms set by the organisms’ circadian clocks, the study shows. The finding, published online October 20 in Nature, may help scientists better understand how plants and animals respond to light-dark cycles.

Most research on circadian clocks has focused on the process by which the biological timekeepers turn genes on and off. But the new study shows that the clocks also govern how molecules of RNA that are transcribed from a gene are spliced together for translation into the gene’s protein product.

“This paper certainly adds a very novel twist,” says Yi Liu, a biologist at the University of Texas Southwestern Medical Center at Dallas who was not involved in the study.

Marcelo Yanovsky, a plant physiologist and geneticist at the IFEVA Institute of Agronomy and the Fundación Instituto Leloir in Buenos Aires, Argentina, and his colleagues searched for genes in the plant Arabidopsis thaliana that cause it to raise its leaves to the light during the day and let them droop back down in the dark. The researchers found a form of the plant that moved its leaves out of sync with the usual 24-hour rhythm, in a 30-hour cycle, and traced the source of the longer cycle to a mutation in the gene PRMT5.

PRMT5 makes an enzyme that adds methyl groups to histone proteins — proteins that form spools on which DNA is wound to fit inside cells. The addition of the chemical tag causes DNA to pack more tightly and shuts down gene activity. The team found that the enzyme also adds methyl groups to proteins involved in cutting and pasting RNA molecules that will later be made into proteins.

The cut-and-paste process is known as alternative splicing, and it allows cells to create different versions of proteins much the way that film editors can splice scenes together to produce movies with alternative endings.