Cover Picture

Abstract

The cover picture shows an illustration of the mechanism of dsRNA-mediated RNA interference (RNAi). RNAi is important for the silencing of transposons and as an antiviral defense mechanism in animals and plants. RNAi is initiated by the presence of double-stranded RNA (dsRNA) in a cell and results in the sequence-specific degradation of mRNAs (7mG=7-methylguanosine 5′ cap, (A)n=polyadenosine 3′ tail) homologous in sequence to the dsRNA. The mediators of target mRNA cleavage are 21–23-nt small interfering RNA duplexes (siRNAs) produced by RNase III cleavage from dsRNA. The siRNAs are characterized by the presence of overhanging 3′ ends and are associated with yet uncharacterized protein(s) (blue and red ellipses). The siRNA–protein complex is referred to as an siRNP (small interfering ribonucleoprotein) particle. The cleavage sites in the dsRNA and the target mRNA are indicated as black triangles. The target RNA cleavage site is displaced by one helical turn relative to the dsRNA cleavage site. An siRNP is an asymmetric complex, and only one of the constituting 21-nt RNA strands is capable of guiding target RNA cleavage. More about the recent advances in understanding the molecular mechanism of RNAi and its biological function can be found in the Minireview by T. Tuschl on p. 239 ff.