Very short RNAs, such as small interfering RNAs (siRNAs) and microRNAs (miRNAs), previously found only in multicellular organisms, have been discovered in a unicellular alga. The discovery, reported in Nature (Molnar A et al Nature 2007 Jun 28;447(7148):1126-9), forces a rethink about the evolutionary history of these molecules. siRNAs and miRNAs have been found in a variety of multicellular organisms including fungi, plants, protozoans and metazoans, where they have been implicated in the control of growth and development. They are also a useful tool in the laboratory, where they can be used to silence gene expression (RNA interference). David Baulcombe, Coordinator of FP6 Integrated Project SIROCCO, and his colleagues have shown that the unicellular alga Chlamydomonas reinhardtii also contains very short RNAs that direct cleavage of their target RNA — just like their higher-plant counterparts. The authors suggest that miRNAs were probably present in primitive eukaryotic cells and that they evolved before multicellularity. This contrasts with the previous theory that suggested that miRNAs evolved together with multicellularity in separate plant and animal lineages.
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