Seeds of a Micro Revolution
Plant studies reveal a slew of miRNAs and some real surprises
By Nicole Johnston

Courtesy of Allison C. Mallory, Whitehead Institute for Biomedical Research

Among structural and regulatory RNAs, the newest kids on the block have
sparked a flurry of breakthrough discoveries in just three years. Tiny RNAs,
called microRNAs (miRNAs), roughly 22 nucleotides long, are processed from
stem-loop transcripts and bind and direct the repression of target mRNAs.
Though both short interfering RNAs (siRNAs) and miRNAs are processed from
double-stranded precursor transcripts by the Dicer nuclease, other events
appear to diverge.

This issue's Hot Papers focus on the discovery of miRNAs in plants. David
Bartel's group at the Whitehead Institute for Biomedical Research and the
Massachusetts Institute of Technology,1,2 and Jim Carrington's group at
Oregon State University,3,4 introduced plant miRNAs, defined the methods to
detect and characterize them, and established a cleavage-mediated silencing
mechanism. In plants, some miRNAs degrade their mRNA targets, whereas the
first miRNAs discovered in animals downregulate translation without
cleavage.

In many ways these discoveries were past due. Tapping plants as these papers
did added to a groundswell of data on miRNAs, their targets, and even their
twisted phenotypes. With dozens of researchers directing efforts at hundreds
of miRNAs and their targets, the story is still unfold.

A PERFECT MATCH Micro RNAs had gone long undetected owing to their tiny size
and lack of open-reading frames. To date, miRNAs are found only in complex
multicellular organisms and are conserved from worms to humans. In the plant
world, they're conserved among such anciently diverged plants as Arabidopsis
and rice.

In 1993, Victor Ambros and colleagues discovered the first 22-nucleotide
RNA, lin-4.5 In 2000, Gary Ruvkun's group found another, let-7.6 Both were
discovered by their mutant phenotypes in Caenorhabditis elegans; lin-4 and
let-7 mutants affected developmental timing and cell fate during the first
and fourth larval stages, respectively.7 The floodgates opened in 2001, when
three groups including Bartel's reported an abundance of miRNAs in C.
elegans through cloning and bioinformatics.

Read the rest at TheScientist
http://www.the-scientist.com/yr2004/sep/feature1_040913.html

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