The influence of the 5΄-terminal nucleotide on AgoshRNA activity and biogenesis: importance of the polymerase III transcription initiation site

Recent evidence indicates that shRNAs with a relatively short basepaired stem do not require Dicer processing, but instead are processed by the Argonaute 2 protein (Ago2). We named these molecules AgoshRNAs as both their processing and silencing function are mediated by Ago2. This alternative processing yields only a single RNA guide strand, which can avoid off-target effects induced by the passenger strand of regular shRNAs. It is important to understand this alternative processing route in mechanistic detail such that one can design improved RNA reagents. We verified that AgoshRNAs trigger site-specific cleavage of a complementary mRNA. Second, we document the importance of the identity of the 5΄-terminal nucleotide and its basepairing status for AgoshRNA activity. AgoshRNA activity is significantly reduced or even abrogated with C or U at the 5΄-terminal and is enhanced by introduction of a bottom mismatch and 5΄-terminal nucleotide A or G. The 5΄-terminal RNA nucleotide also represents the +1 position of the transcriptional promoter in the DNA, thus further complicating the analysis. Indeed, we report that +1 modification affects the transcriptional efficiency and accuracy of start site selection, with A or G as optimal nucleotide. These combined results allow us to propose general rules for the design and expression of potent AgoshRNA molecules.