HOUSTON, June 21, 2004 -- First discovered in abundance three years ago, scientists around the world are racing to find and study microRNAs -- tiny strands of ribonucleic acid (RNA) that regulate gene expression in plants and animals.
It's believed that hundreds of microRNAs exist in each species of plant and animal, but the function of only a few is understood.
According to a new study by biologists at Rice University and the Whitehead Institute for Biomedical Research at the Massachusetts Institute of Technology, one of the first-discovered plant microRNAs -- known as miR164 -- plays a vital role in the proper development of the flowers, leaves, and stems of Arabidopsis.
"We know the miR164 gene is present in some other flowering plants and in rice, which is an indication that it's been conserved over more than 250 million years of evolution," said study co-author Bonnie Bartel, associate professor of biochemistry and cell biology at Rice. "Our study has found that this tiny strand of RNA is a crucial regulatory component of the molecular circuitry that controls basic organ development in the plant."
The study is featured on the cover of the June 22 issue of the journal Current Biology.
The research involved experiments on three strains of Arabidopsis: one normal, or wild-type strain, and two mutant strains created in the lab. In one of the mutants, miR164 was "overexpressed," or produced in far greater quantity than normal. The other mutant expressed an miR164 target gene that was resistant to regulation by miR164, allowing the researchers to observe the consequences of a loss of miR164 regulation.
Researchers found abnormal development of leaves and flowers in both mutants. When miR164 regulation of a target gene was absent, plants produced the wrong number of organs. For example, flowers tended to contain too many petals and too few sepals, the husk-like coverings that protect the flower during budding. When too much miR164 was present, organs tended to fuse. Sepals never fully separated, for example, and stamen -- one the plant's primary reproductive organs -- grew together in a mass.
RNA, or ribonucleic acid, is the chemical that translates the genetic code of DNA into a protein. Structurally, RNA is a polymer consisting of a chain of phosphate, sugar and nucleotides. The most common type of RNA, messenger RNA, contains thousands of nucleotides and is the genetic blueprint that cells use to combine amino acids into proteins.
MicroRNAs are much smaller, typically only about 20 nucleotides long. All microRNAs "down-regulate" the production of certain proteins by interfering with messenger RNA in several ways.
One of the mysteries about microRNAs is why they have been conserved through evolution for so long; they are thought to have first evolved before animals and plants broke out into separate evolutionary tracks. By studying the function of microRNAs in specific species, scientists hope to answer basic questions about how and why they evolved, as well as gain an understanding that can be used in new medical and agricultural technologies.
Arabidopsis, a common genetic model that is one of the most widely studied of all plant species, is the first plant in which microRNAs were discovered, and almost 100 microRNAs have been identified in the species.
The research was supported by the National Institutes of Health, the G. Harold and Leila Y. Mathers Charitable Foundation, the Robert A. Welch Foundation, and the Alexander and Margaret Stewart Trust. Co-authors are David Bartel, associate professor of biology at Whitehead Institute and MIT; Whitehead post-doctoral researcher Allison Mallory; and Rice graduate student Diana Dugas.
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