Researchers discover beneficial jumping gene
Transposons are DNA elements that can multiply and change their location within an organism’s genome. Discovered in the 1940s, for years they were thought to be unimportant and were called “junk DNA.” Also referred to as transposable elements and jumping genes, they are snippets of “selfish DNA” that spread in their host genomes serving no other biological purpose but their own existence.
Now Tokuji Tsuchiya and Thomas Eulgem, geneticists at the University of California, Riverside, challenge that understanding. They report online this week in the Proceedings of the National Academy of Sciences that they have discovered a transposon that benefits its host organisms.
Working on the model plant Arabidopsis, they found that the COPIA-R7 transposon, which has jumped into the plant disease resistance gene RPP7, enhances the immunity of its host against a pathogenic microorganism that is representative of a large group of fungus-like parasites that cause various detrimental plant diseases.
“We provide a new example for an ‘adaptive transposon insertion’ event — transposon insertions that can have beneficial effects for their respective host organisms — and uncover the mechanistic basis of its beneficial effects for plants,” said Thomas Eulgem, an associate professor of plant cell biology and the senior author of the research paper. “While it has been known for a while that transposon insertions can have positive effects for their respective host organisms and accelerate evolution of their hosts, cases of such adaptive transposon insertions have been rarely documented and are, so far, poorly understood.”
The COPIA-R7 transposon affects RPP7 by interfering with the latter’s epigenetic code. In contrast to the well known 4-letter genetic DNA code, which provides instructions for the synthesis of proteins, the “epigenetic code” defines the activity states of genes and determines to what extent their genetic information is utilized. Eulgem explained that the transposition of transposons is typically inhibited by epigenetic silencing signals associated with their DNA. Such epigenetic signals are like molecular “flags” or “tags” that are attached to special proteins, around which DNA is wrapped.
A type of molecular flag, referred to as H3K9me2, prohibits transposons from being active and jumping in their host genomes.
“An exciting aspect of our work is that H3K9me2 signals associated with COPIA-R7 have acquired a completely new meaning in RPP7 and promote the activity of this disease-resistance gene,” said Eulgem, a member of UC Riverside’s Center for Plant Cell Biology. “By modulating levels of this silencing signal in RPP7, plants can adjust the activity of this disease resistance gene.