Representation of a DNA transposon jumping into a gene to make a new intron. Courtesy of Jason Huff.
In the genomes of complex organisms, including humans, gene regions are often interrupted by segments—called introns—that must be removed for proteins to be correctly produced. Biologists were surprised to find these interrupting segments four decades ago. Since then, the origins of introns have become ever more mysterious, as evidence accumulated for an irregular history, with relatively few points in evolution during which a lot of new introns were made. This appears at odds with introns being so ubiquitous in modern genomes.
How are so many introns made? And why did introns accumulate so unevenly? A study published in Nature last week by CNR researchers Jason Huff and Daniel Zilberman, in collaboration with Scott Roy of San Francisco State University, begins to answer these questions.
Huff, a former postdoctoral researcher in the Department of Plant and Microbial Biology (PMB), is director of the QB3 Computational Genomics Resource Laboratory. Zilberman is an associate professor in PMB.
To identify a mechanism to make introns, the authors studied two very distantly related and poorly understood algae that have a lot of fairly new introns. They found that the new introns come from genetic elements, known as DNA transposons, that jump around genomes and can land in genes in such a way that they are removed correctly on the way to protein production. DNA transposons can proliferate rapidly in genomes for a while and then stop, explaining why introns are generated in dramatic episodes. The evolutionary distances between the two organisms and their transposons reveal that the two cases arose independently, suggesting that the mechanism the authors describe could be a general explanation for how introns are made.