MicroRNAs found in unicellular alga

MicroRNAs control gene expression in a single-celled alga, reports a paper in this week's Nature. The finding suggests that microRNAs evolved earlier than previously thought, according to the authors. "This is a very important result," said Scott Poethig of the University of Pennsylvania in Philadelphia, who wasn't an author on the paper. "This is the first single-celled organism in which microRNAs have been discovered."A recent paper in Genes and Development also reported finding microRNAs in the same alga.Previous studies have found short interfering RNAs (siRNAs) in unicellular organisms, but not microRNAs, according to David Baulcombe of the Sainsbury Laboratory in Norwich, UK, senior author of the Nature paper. Some researchers have suggested that microRNAs evolved along with multicellularity, "playing a role in that evolution, perhaps independently in plants and animals," Heriberto Cerutti of the University of Nebraska in Lincoln, who was not involved in the work, told The Scientist. Led by Attila Molnár, also of the Sainsbury Laboratory, the researchers scanned the genome of the unicellular alga Chlamydomonas reinhardtii for potential small RNAs. They used a high-throughput method that allowed them to "look deeper into the small RNA transcriptome than was possible previously," Baulcombe told The Scientist in an Email.Based on a computational analysis of the potential of these small RNAs to fold into microRNA precursors, the authors identified 68 candidate microRNAs. They then used a method developed in the plant Arabidopsis to predict the RNA targets of 18 of these microRNAs. They found evidence that four of the 18 targets were cleaved by the microRNA, as they are in plants. According to the authors, they may have failed to detect cleaved targets of the other 14 microRNAs because the RNA targets may not be expressed in the cells they analyzed, or the cleavage products may not be stable enough for analysis.Another possibility, according to Poethig, is that algal microRNAs do not cleave their RNA targets, as plant microRNAs usually do, but instead interfere with protein translation, a similar process to what happens with most animal microRNAs.The researchers also found that microRNAs in Chlamydomonas exist in two forms. One form, which is the same as that in higher plants, produces a single microRNA from each precursor. The others "have structural characteristics of miRNAs but there are multiple species of short RNA derived from a single precursor," Baulcombe explained. These "are likely evolutionary intermediates in the evolution of miRNAs."With the mechanism of one miRNA molecule for each precursor, it's "as though evolution has worked its way down to finding just the right sequence," Poethig said.Previous researchers likely didn't find microRNAs in single-celled organisms because they looked in organisms with relatively simple genomes, such as yeast, Cerutti said. Chlamydomonas reinhardtii has a much larger, more complex genome. "MicroRNAs might be present in other unicellular organisms that have complex genomes," he told The Scientist.The discovery of microRNAs in algae suggests that they evolved in a unicellular ancestor of both algae and higher plants, Baulcombe said. The short RNAs may have also arisen independently in algae, plants, and animals, he added.Melissa Lee Phillips mail@the-scientist.comLinks within this article:D. Steinberg, "MicroRNA shows macro potential," The Scientist, June 16, 2003. http://www.the-scientist.com/2003/6/16/22/1/A. Molnár et al., "MiRNAs control gene expression in the single-cell alga Chlamydomonas reinhardtii," Nature, published online May 30, 2007. http://www.nature.com/nature/journal/vaop/ncurrent/abs/nature05903.htmlJ.M. Perkel, "MicroRNA evolution put to the test," The Scientist, October 30, 2006. http://www.the-scientist.com/news/display/25713/Scott Poethig http://www.bio.upenn.edu/faculty/poethig/T. Zhao et al., "A complex system of small RNAs in the unicellular green alga Chlamydomonas reinhardtii," Genes and Development, May 15, 2007. http://www.the-scientist.com/pubmed/17470535S. Jaffe, "To siRNA with love," The Scientist, April 26, 2004. http://www.the-scientist.com/article/display/14634/David Baulcombe http://www.sainsbury-laboratory.ac.uk/dcb/E. Allen et al., "Evolution of microRNA genes by inverted duplication of target gene sequences in Arabidopsis thaliana," Nature Genetics, December 2004. http://www.the-scientist.com/pubmed/15565108Heriberto Cerutti http://psiweb.unl.edu/cerutti/home.htmlR. Schwab et al., "Specific effects of microRNAs on the plant transcriptome," Developmental Cell, April 2005. http://www.the-scientist.com/pubmed/15809034A.A. Millar, P.M. Waterhouse, "Plant and animal microRNAs: similarities and differences," Functional and Integrative Genomics, July 2005. http://www.the-scientist.com/pubmed/15875226

MicroRNAs found in unicellular alga

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