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Viroids: introns on the run?

Viroids: Introns on the run?

It can be satisfying when two conundrums fit together -- they may just form one big puzzle, but at least some loose ends have been tied together.

Viroids are the smallest infectious particles know, consisting solely of naked RNA. So far as scientists have been able to tell after years of study, the RNA doesn't code for a single protein -- yet somehow this "silent" genetic material can cause disease in the plant hosts. Introns are silent, too -- bits of genetic material that can be seen in most DNA, only to be snipped out of the complementary RNA before it is translated into proten (SN:5/3/86,p.280). Now comes evidence for an idea first proposed in 1979: that viroids are "escaped" introns.

According to Gail Dinter-Gottlieb, there is a striking similarity in the nucleic acid sequences, and possibly in the structures, of viroids and a certain class of introns. These "group 1" introns can be found in both plants and animals, and in all three cellular organelles (in chloroplast, nuclear and mitochondrial RNA); as a group, they are defined by a shared, 16-nucleotide consensus sequence. In the September PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol.83, No.17), Dinter-Gottlieb reports that all of the viroids so far sequenced contain the consensus sequence as well. (Dinter-Gottlieb, now at Drexel University in Philadelphia, did the work at the University of Colorado in Boulder.)

There are other nucleotide sequences, called boxes, that are also more or less conserved within the group of introns. Complementary base-pairing within these boxes forces the intron RNA into its 3-dimensional configuration. The viroids contain the boses, in the same order as in the introns. "When the boxes in [one viroid] are paired, a structure is generated in the viroid which is strikingly similar to that of group 1 introns," Dinter-Gottlieb writes. Previous studies have reported a rod-like shape for viroids, but Dinter-Gottlieb speculates that they may fold more elaborately when stabilized by cellular proteins.

The evidence is strong for a close relationship between viroids and introns, Dinter-Gottlieb says, though it remains unclear whether viroids evolved from introns, or merely share a common ancestor molecule. However, she adds, at least on e group 1 intron shares yet other viroid sequences, stretches of RNA responsible for modulating the severity of infection. The similarity, she told SCIENCE NEWS, leads to the question of whether introns might be pathogenic. Perhaps, she speculates, some renegade intron escaped the normal regulatory processes of the cell, and took up an infectious "life"-style.
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Title Annotation:smallest infectious particles known
Publication:Science News
Date:Oct 4, 1986
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