July 15, 1998 ATHENS, Ga. -- Some viruses are easy. They run their infective course, do little damage and then look for another host. Medical researchers have found ways to treat the effects of such viruses by eliminating symptoms or hurrying them on their way. Other viruses are killers.
Especially difficult has been a class called retroviruses, which includes HIV, the cause of AIDS. These viruses are the quick-change artists of the microbial world and outrun both the immune system and medicines designed to fight them. And important in understanding retroviruses are retrotransposons -- pieces of DNA closely related to retroviruses.
Now, researchers at the University of Georgia have uncovered intriguing new clues about the evolution of retrotransposons in a genome -- evidence that could serve as a model system for understanding why retroviral elements evolve so quickly.
"This is the first study to show that transposons exchange genetic information," said Dr. John McDonald, head of the genetics department. "Our study indicates that invading retroviruses may pick up genetic information from other retroelements already present in the genome and thereby evolve at a faster rate."
The study, with Dr. King Jordan, a recent graduate in genetics at UGA, was just published in the July issue of the Journal of Molecular Evolution.
Until the 1970s, scientists thought transposons were simply pieces of "selfish" DNA that roamed around in the cellular world. It soon became clear, however, that retroelements are pervasive. Researchers then understood that retroelements must have a crucial role in the functioning of plants and animals if they had been conserved over thousands of millennia.
Jordan and McDonald chose to study the evolution of yeast retroelements because the genetic sequence of the Saccharomyces cervisiae genome was the first to be completed.
"The ability to compare the sequences of all retroelements within a genome provides a unique opportunity to study the elements' evolution," said McDonald.
Little is known so far about how retroviral elements establish themselves and evolve withing the host genome. Analysis of the molecular variation within and between families of retroelements is essential to understand how these elements work and evolve.
There are five families of yeast retroelements referred to as "Ty elements." Jordan and McDonald analyzed the genomic complement of two families, Ty1 and Ty2. These families are closely related and are classified as "long terminal repeat (LTR)" retrotransposons. (Terminal repeats are particular sequences of nucleotides that appear on both ends of a DNA or RNA molecule.)
Their analysis concluded that the high level of identity between the LTRs of Ty1 and Ty2 showed that they have "recently transposed" -- that is, they have recently moved around the host genome.
"Perhaps most interesting , however, is that these elements can readily exchange genetic information." said Jordan.
The type of analysis carried out by Jordan and McDonald will also be useful in human beings when the entire human genome or gene map is completed in the next few years. Scientists know that in yeast less than 5 percent of the genome is made up of retroelements, while in lilies, retroelements make up an astounding 98 percent. In humans, retroelements account for about 35-40 percent of the genome. When the human gene map is completed, studies such as the one done by Jordan and McDonald, could help us understand how retroviruses are able to evolve so rapidly and thus outrun the host immune system.
"Understanding how new retroviruses, such as HIV, evolve will move us closer to being able to control retroviral-based diseases like AIDS," said Jordan.
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