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Screen Provides Clues About Mobile Elements In the Rice Genome

NEW YORK (GenomeWeb News) – A paper appearing online today in Science is offering new insights into how a group of small transposable elements use other transposons to help them move around the rice genome.

Researchers from the Universities of Georgia, Toronto, and Texas used a yeast screen to investigate a type of mobile genetic elements dubbed "miniature inverted repeat transposable elements," or MITEs. Their findings suggest that the MITEs they tested move around the rice genome by relying on transposase proteins affiliated with mobile elements called Osmars.

"These findings suggest that MITEs achieve high transposition activity by scavenging transposes encoded by distantly related and self-restrained autonomous elements," senior author Susan Wessler, a plant science researcher at the University of Georgia, and her co-authors wrote.

In plants, MITEs are classified as either Tourist or Stowaway MITEs, depending on their sequence and structure. The elements are common in the rice genome, where more than 22,000 Stowaway MITEs have been detected, the authors explained. In addition, phylogenetic analyses hint that various MITE sub-groups in rice have come about through the amplification of just a handful of elements to produce hundreds or thousands of MITEs.

But despite being peppered throughout plant genomes in high numbers, MITEs are typically non-autonomous, meaning they lack coding capacity or the ability to replicate and move themselves around the genome. Instead, they are thought to rely on transposases — proteins that promote the replication and movement — belonging to other transposons.

"[B]ecause none of these MITEs are active, their origin, success, and source of transposase have been a mystery," Wessler and her co-authors wrote.

In an effort to gain a better understanding of Stowaway type MITEs in the rice genome, Wessler and her team developed a modified yeast reporter assay to look for interactions between these MITEs and Osmars — suspected of containing transposases used by MITEs.

The researchers transformed yeast with two different plasmids — one containing one of seven Osmar transposases and another containing one of 24 different Stowaway MITEs — and looked for evidence of interactions between them.

Indeed, the team found evidence that rice MITEs not only interact with Osmar transposes, but also seem to use them to facilitate their movement — a process known as cross-mobilization.

The researchers' subsequent binding and site-directed mutagenesis experiments provided additional information about some of the specific Osmars involved, as well as some of the details of these interactions.

"Our data support a model for the cross-mobilization of Stowaway elements by Osmar transposase and suggest how Stowaway MITEs may arise and amplify in the genome," the team concluded. "Because MITEs are widespread in eukaryotes, the fundamental principles outlined herein may be applicable to a broad range of organisms."