Molecular Biology

Despite the great promise that short interfering RNA (siRNA) induced RNAi responses hold as a therapeutic modality, due to their size (~15 kDa) and high negative charge (Bumcrot et al., 2006), siRNAs have no bioavailability and require a delivery agent to enter cells (Figure 1). TAT peptide transduction domain (PTD) has been developed as an agent that mediates cellular delivery of macromolecular therapeutics that otherwise lack bioavailability, making it a tantalizing candidate for siRNA delivery (Farkhani et al., 2014). Unfortunately, when conjugated to TAT PTD, the presence of 40 negative phosphodiester backbone charges on siRNA neutralizes the cationic PTD resulting in aggregation and poor cellular delivery (Meade and Dowdy, 2007). In light of this, we synthesized a neutral RNAi trigger, termed siRiboNucleic Neutrals, for conjugation to TAT PTD (Meade et al., 2014). In brief, the negatively charged phosphodiester backbone was neutralized by synthesis with bio-reversible phosphotriester protecting groups which are specifically converted into charged phosphodiester bonds inside of cells by the action of cytoplasmic restricted thioesterases resulting in a wild type siRNA that can induce RNAi responses. Here we describe the conjugation and cellular delivery of siRNN oligonucleotides with TAT PTD delivery domain (DD) HyNic peptides.

Compared to the recent dramatic growth in the numbers of genome-wide and functional studies of complex non-coding RNAs, mechanistic and structural analyses have lagged behind. A major technical bottleneck in the structural determination of large RNAs and their complexes is preparation of diffracting crystals. Empirically, a vast majority of such RNA crystals fail to diffract X-rays to usable resolution (~4 Å) due to their inherent disorder and non-specific packing within the crystals. Here, we present a protocol that combines post-crystallization cation replacement and dehydration that dramatically improved the diffraction quality of crystals of a large gene-regulatory mRNA-tRNA complex. This procedure not only extended the resolution limit of X-ray data from 8.5 to 3.2 Å, but also significantly improved the quality of the data, enabling de novo phasing and structure determination. Because it exploits the general importance of counterions and solvation in RNA structure, this procedure may prove broadly useful in the crystallographic analyses of other large non-coding RNAs.

5’ end-labeled RNA molecules are useful substrates to analyse the endo- and exonucleolytic activities of various ribonucleases. Here two protocols are given to synthesize P³² labeled RNAs with a 5’ PPP or 5’ P moiety. 5’ exoribonucleases generally do not work on 5’ PPP RNA and require a 5’ P substrate. The activity of certain endoribonucleases like Escherichia coli (E. coli) RNase E or Bacillus subtilis (B. subtilis) RNase Y can be stimulated by a 5’ P moiety.