Micro-RNAs (miRNAs) have been found in plants, animals and microorganisms, but very few have been functionally characterized. In the September 20 Science, Llave et al. describe a specific function for the Arabidopsis miRNA 39, which accumulates in inflorescence tissue and is produced from an intergenic region in chromosome III (Science, 297:2053–2056, Septemeber 20, 2002).

The miRNA 39 sequence is perfectly complementary to an internal sequence in mRNAs encoding three members of the Scarecrow-like (SCL) transcription factor family. Llave et al. show that miRNA 39 guides the internal cleavage of target SCL mRNAs; they propose that miRNA 39 activity might involve the RISC complex (RNA-induced silencing complex) used in RNA silencing.

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