Introduction

RNA silencing, a nucleotide sequence-specific RNA degradation mechanism that results in the suppression of gene expression, has emerged over the past decade as a topic of interest for the genetic manipulation of eukaryotes. This process, which is manifested in organisms ranging from protozoa to vertebrates, is triggered by double-stranded RNA (dsRNA) molecules, which are processed into 21–25 nucleotide (nt)-long RNA duplexes by an RNaseIII enzyme named Dicer. These small interfering RNAs (siRNAs) are incorporated into a multiprotein complex, the RNA-induced silencing complex (RISC), which specifically degrades all mRNA sharing sequence identity with the siRNAs. (For recent reviews see Cerutti, 2003; Denli and Hannon, 2003.)

The dsRNA molecules that trigger the silencing response are found naturally in an eukaryotic cell as replicative intermediates upon virus infection, or during the replication process of a transposable element. Experimentally, RNA silencing can be triggered by the deliberate introduction of dsRNA molecules or inverted repeat transgenes, the latter inducing gene silencing through transcription into hairpin dsRNAs in the nucleus. However, transgenes transcribing only sense RNA are also able to activate the silencing mechanism (Meins, 2000). Exactly how these sense transgenes are able to produce dsRNA molecules is still an open question, although it has been proposed that abnormally processed RNA (“aberrant” RNA) generated from these transgenes is the signal that triggers the silencing mechanism.