1. Sainsbury Laboratory, John Innes Centre, Norwich NR4 7UH, UK
2. University of East Anglia, School of Computing Sciences, Norwich NR4 7TJ, UK

Correspondence to: David C. Baulcombe¹ Correspondence and requests for materials should be addressed to D.C.B. (Email: david.baulcombe@tsl.ac.uk).

Abstract

MicroRNAs (miRNAs) in eukaryotes guide post-transcriptional regulation by means of targeted RNA degradation and translational arrest¹. They are released by a Dicer nuclease as a 21–24-nucleotide RNA duplex from a precursor in which an imperfectly matched inverted repeat forms a partly double-stranded region. One of the two strands is then recruited by an Argonaute nuclease that is the effector protein of the silencing mechanism. Short interfering RNAs (siRNAs), which are similar to miRNAs, are also produced by Dicer but the precursors are perfectly double-stranded RNA. These siRNAs guide post-transcriptional regulation, as with miRNAs, and epigenetic genome modification. Diverse eukaryotes including fungi, plants, protozoans and metazoans produce siRNAs^{2, 3, 4, 5} but, until now, miRNAs have not been described in unicellular organisms and it has been suggested that they evolved together with multicellularity in separate plant and animal lineages⁶. Here we show that the unicellular alga Chlamydomonas reinhardtii contains miRNAs, putative evolutionary precursors of miRNAs and species of siRNAs resembling those in higher plants. The common features of miRNAs and siRNAs in an alga and in higher plants indicate that complex RNA-silencing systems evolved before multicellularity and were a feature of primitive eukaryotic cells.

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