Like the deleted scenes from the movie, some sequences of our genes end up in the courtroom floor, and cells do not use them to make proteins. Now, two studies show that these segments, known as introns, help to survive yeast during difficult times. The research reveals another possible function of a type of DNA that scientists felt was useless.
"They are very strong, very convincing and very interesting results," says evolutionary molecular biologist Scott Roy of the State University of San Francisco in California, which was not related to the studies. The investigation "opens up a whole new paradigm that introns could be doing". He also responds to the question of why yeast has maintained what was previously considered "garbage DNA," says yeast microbiologist Guillaume Chanfreau of the University of California, Los Angeles.
Introns are prevalent in plants and fungi, as well as in humans and other animals, each of our nearly 20,000 genes has an average of eight. When one of our cells begins to make a protein of a particular gene, enzymes generate a copy of the RNA that includes introns. Then, the cell clears the introns of the RNA and seals the remaining parts of the molecule again. This molecule of ARN edited serves as seed guides to build the protein.
The elimination of introns requires a lot of energy, and a complex set of molecular scissors, which suggests sequences evolve to perform specific functions. After first deleting them as junk, researchers have recently started to identify some of these roles. For example, introns of some genes can help control how many of the corresponding proteins make the cell.
But in yeast of bread, an organism that has exiled most of its introns (it only has 295 for about 6000 genes), the functions of most sequences are dim. Scientists who eliminated individual introns, for example, found that, in most cases, fungi were not spoiled.
However, researchers have generally not looked at yeast in conditions that would face nature, where it could withstand periods of food shortages that do not occur in the laboratory. To determine what happens during deprivation, RNA biologist, Sherif Abou Elela, from the University of Sherbrooke in Canada, and his colleagues systematically eliminated introns from yeast, producing hundreds of strains, each of which lacked all introns of a gene. The researchers created combinations of these modified strains alongside normal fungi.
When the food was scarce, most of the strains that were missing for the intron were rapidly extinguished, according to today's computer Nature. They could not compete with normal yeasts. However, in cultures with more nutrients, the altered yeast had the advantage. "If you're in good times, it's a burden" to have introns, says Abou Elela. "In bad times, it is profitable".
Molecular biologist David Bartel of the Massachusetts Institute of Technology at Cambridge and colleagues obtained independent results with similar results. They measured the amounts of different RNA molecules in yeast cells, and expected that most introns quickly deteriorate after being deleted from their chain of parental RNAs. But as they report today in another article Nature, he noticed that a large number of introns accumulated in cells grew in crowded crops.
"It was incredibly strange," says Bartel Jeffrey Morgan's postgraduate student, now a molecular biologist at the University of Utah in Salt Lake City. Like the team at Abou Elela, the Bartel group found that introns helped the yeast under coercion but the cells were injured in more favorable conditions. Scientists suspect that introns help stress fermentation grow.
Although it is still unclear how these introns provide their benefits, the two studies suggest similar mechanisms. As the yeast's environment becomes severe, the introns become more abundant and could stop the molecular shears that normally dump RNAs, reducing the synthesis of some proteins and allowing the cells to conserve its resources. This may seem like a complicated process, but "evolution does not always choose the easiest solution," explains Bartel.