A biochemical framework for RNA silencing in plants Guiliang Tang, Brenda J. Reinhart, David P. Bartel, and Phillip D. Zamore have identified a biochemical pathway for RNA silencing in plants. They found that wheat germ extract contains Dicer-like enzymes that convert double-stranded RNA (dsRNA) into small interfering RNAs (siRNAs) and an RNA-dependent RNA polymerase (RdRP) that can convert single-stranded RNA into dsRNA. In plant embryos, an endogenous microRNA (miRNA) acts as a small interfering RNA, guiding an endonuclease to cleave target mRNA. This supports the view that plant miRNAs direct RNAi and that miRNA-specified mRNA destruction is important for plant development. RNA silencing is a broad phenomenon that includes RNA interference (RNAi) in animals, quelling in fungi, and posttranscriptional gene silencing (PTGS) in plants. The unifying features of RNA silencing are the production of small (21–26 nt) RNAs that act as specificity determinants for downregulating gene expression and the requirement for one or more members of the Argonaute family of proteins. In animals, small RNAs are generated by members of the Dicer family of double-stranded RNA (dsRNA)-specific endonucleases. Dicer family members are large, multidomain proteins that contain putative RNA helicase, PAZ, two tandem ribonuclease III (RNase III), and one or two dsRNA-binding domains. The tandem RNase III domains are believed to mediate endonucleolytic cleavage of dsRNA into small interfering RNAs (siRNAs), the mediators of RNAi. In plants, PTGS initiated by transgenes that overexpress an endogenous mRNA also requires a putative RdRP, SGS2 (SDE1), although transgenes designed to generate dsRNA bypass this requirement. Similarly, silencing induced by viruses replicating through a dsRNA intermediate (virus-induced gene silencing, VIGS) does not require SGS2. Dicer in animals and CARPEL FACTORY (CAF) in plants also generate microRNAs (miRNAs), 20–24-nt, single-stranded noncoding RNAs thought to regulate endogenous mRNA expression. miRNAs are produced by Dicer cleavage of stem-loop precursor RNA transcripts (pre-miRNAs); the miRNA can reside on either the 5' or 3' side of the double-stranded stem. In plants, miRNAs have far greater complementarity to cellular mRNAs and have been proposed to mediate target RNA cleavage via an RNAi-like mechanism. The authors present evidence that wheat germ extracts recapitulate many of the key features of RNA silencing in plants. Using this in vitro system, they showA biochemical framework for RNA silencing in plants Guiliang Tang, Brenda J. Reinhart, David P. Bartel, and Phillip D. Zamore have identified a biochemical pathway for RNA silencing in plants. They found that wheat germ extract contains Dicer-like enzymes that convert double-stranded RNA (dsRNA) into small interfering RNAs (siRNAs) and an RNA-dependent RNA polymerase (RdRP) that can convert single-stranded RNA into dsRNA. In plant embryos, an endogenous microRNA (miRNA) acts as a small interfering RNA, guiding an endonuclease to cleave target mRNA. This supports the view that plant miRNAs direct RNAi and that miRNA-specified mRNA destruction is important for plant development. RNA silencing is a broad phenomenon that includes RNA interference (RNAi) in animals, quelling in fungi, and posttranscriptional gene silencing (PTGS) in plants. The unifying features of RNA silencing are the production of small (21–26 nt) RNAs that act as specificity determinants for downregulating gene expression and the requirement for one or more members of the Argonaute family of proteins. In animals, small RNAs are generated by members of the Dicer family of double-stranded RNA (dsRNA)-specific endonucleases. Dicer family members are large, multidomain proteins that contain putative RNA helicase, PAZ, two tandem ribonuclease III (RNase III), and one or two dsRNA-binding domains. The tandem RNase III domains are believed to mediate endonucleolytic cleavage of dsRNA into small interfering RNAs (siRNAs), the mediators of RNAi. In plants, PTGS initiated by transgenes that overexpress an endogenous mRNA also requires a putative RdRP, SGS2 (SDE1), although transgenes designed to generate dsRNA bypass this requirement. Similarly, silencing induced by viruses replicating through a dsRNA intermediate (virus-induced gene silencing, VIGS) does not require SGS2. Dicer in animals and CARPEL FACTORY (CAF) in plants also generate microRNAs (miRNAs), 20–24-nt, single-stranded noncoding RNAs thought to regulate endogenous mRNA expression. miRNAs are produced by Dicer cleavage of stem-loop precursor RNA transcripts (pre-miRNAs); the miRNA can reside on either the 5' or 3' side of the double-stranded stem. In plants, miRNAs have far greater complementarity to cellular mRNAs and have been proposed to mediate target RNA cleavage via an RNAi-like mechanism. The authors present evidence that wheat germ extracts recapitulate many of the key features of RNA silencing in plants. Using this in vitro system, they show