Two classes of short interfering RNA in RNA silencing.
RNA silencing is a eukaryotic genome defence system that involves processing of double-stranded RNA (dsRNA) into 21-26 nt, short interfering RNA (siRNA). The siRNA mediates suppression of genes corresponding to the dsRNA through targeted RNA degradation. In some plant systems there are additional silencing processes, involving systemic spread of silencing and RNA-directed methylation/transcriptional suppression of homologous genomic DNA. We show here that siRNAs produced in plants from a green fluorescent protein (GFP) transgene are in short (21-22 nt) and long (24-26 nt) size classes, whereas those from endogenous retroelements are only in the long class. Viral suppressors of RNA silencing and mutations in Arabidopsis indicate that these classes of siRNA have different roles. The long siRNA is dispensable for sequence-specific mRNA degradation, but correlates with systemic silencing and methylation of homologous DNA. Conversely, the short siRNA class correlates with mRNA degradation but not with systemic signalling or methylation. These findings reveal an unexpected level of complexity in the RNA silencing pathway in plants that may also apply in animals.
The Sainsbury Laboratory, John Innes Centre, Colney Lane, Norwich NR4 7UH, UK
Andrew
A
Hamilton
Olivier
O
Voinnet
Louise
L
Chappell
David
D
Baulcombe
D000222:Adaptation, Physiological; D016960:Agrobacterium tumefaciens / Q000235:genetics; D017360:Arabidopsis / Q000235:genetics; D017796:Caulimovirus / Q000235:genetics; D020868:Gene Silencing; D017930:Genes, Reporter; D005814:Genes, Viral; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins / Q000096:biosynthesis; D018515:Plant Leaves / Q000378:metabolism; D030821:Plants, Genetically Modified; D011401:Promoter Regions, Genetic; D012330:RNA, Double-Stranded / Q000235:genetics*; D018749:RNA, Plant / Q000145:classification* / Q000502:physiology; D034741:RNA, Small Interfering; D022661:RNA, Untranslated / Q000145:classification* / Q000502:physiology; D012367:RNA, Viral / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D018626:Retroelements / Q000235:genetics; D014026:Nicotiana / Q000235:genetics; D019076:Transgenes
D008164:Luminescent Proteins; D012330:RNA, Double-Stranded; D018749:RNA, Plant; D034741:RNA, Small Interfering; D022661:RNA, Untranslated; D012367:RNA, Viral; D011993:Recombinant Fusion Proteins; D018626:Retroelements; D049452:Green Fluorescent Proteins