Adding some complexity to the seemingly simple life of a single-celled organism, researchers have found that a green alga uses snippets of RNA to control its genes. The finding, the first "microRNAs" outside of the multicellular world of plants and animals, indicates that simple organisms can regulate genes in ways similar to their more advanced counterparts.
MicroRNAs, or miRNAs, are chains of about two dozen nucleotides--peewees to most RNAs in a cell--bent into a hairpin shape. In the last six years, researchers have discovered that these elements control the expression of genes in animals and plants (Science, 26 October 2001, p. 797). In both kingdoms, the miRNA interacts with the protein-coding messenger RNA strand, but in different ways: In animals, it stops the template from getting read past a certain point, whereas in plants it cleaves the strand. MiRNAs were known to control the development of animals and plants, but because they hadn't been found in simpler creatures, it wasn't clear if they emerged after multicellular organisms evolved.
Yijun Qi, a molecular biologist at National Institute of Biological Sciences in Beijing, China, decided to hunt for microRNAs in the green alga Chlamydomonas reinhardtii. Known as Chlamy to researchers, this alga's combination of traits--it has a cell wall and chloroplasts, but also an eyespot and pair of flagella, and switches between sunlight and carbon for food--has made it a popular study subject for decades.
Qi and his team grew Chlamy in the lab and isolated the thousands of short RNA strands it naturally produced. They then analyzed each RNA's sequence, picking out ones that could be miRNAs based on the shapes predicted by their sequences and other features shared by plant and animal miRNAs. In a paper published online today in the journal Genes and Development, the team reports finding 21 candidate miRNAs. The researchers also report finding similar, but not identical, miRNA sequences in three other species of green algae.
Additional experiments showed that Chlamy uses miRNAs just like plants do, by cleaving a lengthy RNA strand at just the right spot before it's used as a protein template. But because none of Chlamy's miRNA sequences resemble those of plants or animals, Qi speculates that each group of gene-controllers arose independently in the three lineages.
Plant geneticist David Stern of Cornell University agrees, adding that the team's database of Chlamy RNA sequences will likely provide a major resource for RNA research in the organism. The finding of miRNAs in organisms as simple as Chlamy makes sense, Stern says, as "much of biology turns out to be run by the RNA world."